The exile happened because of a past TRAUMA.
I mean not exiled for a crime other than Scar's. Fanfiction is fun fun fun! But the bottom line is; in movie making if anything gets complicated, it would be somewhat or extremely directly explained - NOT hidden behind hints.
Even the director of TLK2 says "the seeds of it are in the first film". (In the Making of documentary "Proud of Simba's Pride".) And the last time I checked, no crimes happen in that film apart from the one committed by Scar. And because the second film did not specify any crime other than the one Scar committed, that's the only one happened.
To back up this even more... There is NO need for a new crime to explain anything in TLK2.
If you bother to give any serious thoughts to the depth and number of the emotional wounds and traumas Scar had given to Simba, and the fact that Simba was not mentally a grown up when he reclaimed the throne, and how Simba learned the truth about Scar probably a very short time before he met Zira & co. - there lies more than enough reason for him to exile Zira and along with her everyone else who simply reminded him of Scar and were more or less with Zira, and thus potential danger. No crime was needed for Simba's choice.
Not fair towards Zira & co.? Very unpleasent to think about? Suck it up! Because life hadn't been fair to Simba! Life had been unpleasent to Simba! Simba was scared! And immature! How could you possibly demand him to be able of acting any differently?
Yes, Simba said "You can't turn your back on them." But of course Simba would say that. He was traumatized! Do you know what the concept of trauma is? The following quotes are from English Wikipedia.
"Psychological trauma is a type of damage to the psyche that occurs as a result of a traumatic event. When that trauma leads to posttraumatic stress disorder, damage may involve physical changes inside the brain and to brain chemistry, which changes the person's response to future stress."
Hence, Scar causing him deep traumas of which the latest was very fresh and came together with all the earlier, so he responds to the stress of kingship and parenthood differently than he probably otherwise would've.
"A traumatic event involves a single experience, or an enduring or repeating event or events, that completely overwhelm the individual's ability to cope or integrate the ideas and emotions involved with that experience. The sense of being overwhelmed can be delayed by weeks, years or even decades, as the person struggles to cope with the immediate circumstances."
Simba's traumatic events:
- At the age of 10 (or so) Helplessly watching his daddy fall to his death.
- A few moments later, trying to wake his daddy with no success, having to deal with this situation all alone.
- Cuddling in his dead father's arms, crying a river, he already blames himself for what happened. ("I didn't mean for it to happen.")
- His uncle Scar comes to this very situation and literally blames Simba. ("What have you done?" "Without you he'd still be alive." "And when your mother hears about this!")
- Simba considers him a friend and chooses an exile for life. A young child leaving his mother, friends and home forever, with the blame of his daddy's death on his shoulders and fresh in his heart, having no place to go and no hope.
- Wanders in a blazing sun/cold nights at a desert for probably couple of days, with no food, no water, no hope - likely wishing to die.
- For years and years he grows up with this pain, hurt and blame. Hakuna Matata probably saved his sanity. But the darkness was always there, he obviously could not completely push it aside.
- When he finally returns to face his ghosts, Scar forces him to confess his responsibility and almost kills him.
- But just before he was to do that, he reveals that HE killed Mufasa. Becoming the ultimate traumatic event seeing to all the above mentioned situations and emotions being because of Scar.
- As if this wasn't enough, Scar threw in how he could not be trusted at all. Simba has mercy on him by deciding to just exile him but Scar throws fire sparks into his eye and tries to kill him again.
Simba's feeling of being overwhelmed obviously was not delayed as TLK2 makes clear that Scar, his crimes and how Simba felt about them, were the reason for his choices with the Outsiders.
"Psychological trauma can lead to serious long-term negative consequences that are often overlooked even by mental health professionals: "If clinicians fail to look through a trauma lens and to conceptualize client problems as related possibly to current or past trauma, they may fail to see that trauma victims, young and old, organize much of their lives around repetitive patterns of reliving and warding off traumatic memories, reminders, and affects."
Zira and everyone who looked like her/Scar fall into the category of reminders which Simba warded off by exiling the lot. But of course the reason was not simply that selfish but also to protect his family from potential danger. He relived his traumas in his dreams, one of the times shown in the film. The affects being paranoid attitude towards those somehow closely related to Scar, ("You can't turn your back on them!"), and obsessive desire to protect his first-born and only child who appears to be just like he was when he was young (at which point his traumas started to build.)
But there were other characters too hinting towards a crime!
Zazu? "Nothing there but back-stabbing, murderous Outsiders!"
But judging by that he recommended Simba to NOT pay his debts to Kovu even though Mufasa's law said he should, and calling Kovu "Riff Raff" - even though Kovu was as innocent as a baby to whatever drama their kingdom had seen in the past. As in he didn't even stop to consider that matter. Thus, obviously, Zazu's attitude towards any Outsider wasn't fair while was passionate, low and negative. Resulting in that he painted the Outsiders much blacker than they truly were. But his cause was good; he tried to prevent a precious little girl from ever going to the Outlands.
Kovu? "I have left the Outsiders. I'm a rogue. Judge me now for who I am. Or am I to be blamed for a crime I didn't commit?"
Now if the refered crime was other than Scar's, it would have been specified in someone's words in that scene or later in the film. Kovu was just being smart. He pulled the right string to wiggle his way into the pride - by poking a at the right spot in Simba's traumatized psyche; reminding Simba that he had banished everyone for a crime they had not committed as well as how he had no valid reason to decline his request to join. As it was quite obvious from the prosper of the kingdom that Simba tried to be good and fair king.
Also, when Simba growls/roars at this and seems unable to pass a positive/agreeing judgmnent, Nala speaks up. She says; "Simba, you owe him your daughter's life." Listen to her tone of voice and look at her expression as she said that and remark he put the most emphasis on the word "life". Even though Zazu interpret it as just about owing something in general, Nala certainly meant to say that Simba shouldn't be that harsh on Kovu because of the reason of the debt clearly over-weighting whatever reason Simba felt like not letting any Outsider in and thus exiled them all years earlier. Which in this case was the mere hatred and mistrust of Scar-related lions.
Simba would've been a total, utter emotional mess and nutcase and very unstable ruler if he had lost his only son/child too, on top of all the loss and traumas Scar had caused him. Especially if he'd lost this child to a murder by Scar-related lion. And Nala would not have so passionately and determinedly reasond with "he saved your daughter's life" if the person in talks had grown up with her son's murderer/suspected murderer or with a criminal in any way. Instead she would've done what Simba did: asked Kovu why he saved her. And the reason being "I want to join your pride" would not be enough to gain trust, quite the contrary in fact. But to Nala, it obviously did not cause suspect. Because the reason for Kovu having been exiled in the first place was not his mother but Scar's crime and Simba's hatred, trauma and prejudice - to which Nala can not relate to on deep enough level as she was not related to Scar, had not lost a close parent because of Scar, had not suffered years of self-blame because of Scar and had not almost been killed by Scar.
Scar, Scar, Scar! One guy CAN single-handedly cause that much trouble and hurt, long term affects to an entire community through abusing its leader long and deeply enough.
So I really don't think there was any crime commited by anyone but Scar and there most certainly was no siblings to Kiara. Because the film didn't say there was and there is absolutely no need for any. And because Simba's Pride's makers have officially confirmed that in their story the cub is Kiara. In this official interview they say "It starts off where the first film left us with a twist - you think it's a king but it's a queen!" And remark also that he says that we THINK it's a boy, not that it actually was in the film itself. But the point is, one of the animators have confirmed that in their story Kiara IS the cub at the end of the first film.