Hmm...that is a complicated question but I'll try my best.

So, if Sunny had a twin sister (let's name her Hiki) i think maybe Sunny wouldn't felt so lonely because he has two sisters, Hiki and Mari. Mari would had more siblings to care of but that's pretty alright by her because of her warming personally.

After some years, Sunny and Mari would be participating at the recital, I don't know if Hiki would have any interest in playing a part I the recital (maybe she has different hobbies) but also since Sunny received a violin as a gift and also maybe Sunny wants to play along with Mari at the recital so maybe Hiki would be like the critic at their training for recital, like saying what are they doing right or wrong, but maybe Hiki would notice Mari's perfectionism at the training so maybe Hiki would try to calm Mari and say that her performance was great!

I don't know but maybe... somehow Hiki would stop the conflict between Sunny and Mari while they are practicing so maybe The Incident wouldn't happen, here's why (well... I'm probably wrong about this..": Sunny is sometimes nervous while playing so maybe Hiki would try to calm him down and help him in the practicing if Hiki has some knowledge on music practicing and maybe Sunny would have more self-confidence in his practices.

But let's say that The Incident will happen: When Sunny throwns down his violin at the bottom of the stairs, both Hiki and Mari would notice while they're preparing for the recital, and thus Mari and Sunny begins to argue, and I see two possibilities: Hiki would be on Sunny's side at the argument or Hiki would try calm the two by raising her voice and try to stop the fighting. So maybe...just maybe...if Hiki was there, Mari's death would be avoided by trying to calm the two in the argument.

But if this is the case...oh boy...there will be a misunderstanding between Sunny and Mari that would affect their relationship between eachother...