I feel like a fanfic wouldn't have for-real autism in it. OP's sister sounds like a serious autist, a genuine autist, not the pretend autist you usually encounter on Yas Forums.
Imouto's
Joshua Clark
Xavier Gonzalez
I have a brother who is autistic, and this OP's sister seems eerily similar to my brother.
Jeremiah Ortiz
...Imouto's what?