Oh Hun.. its very difficult to explain, i have twins who are now nine Kylie and Thomas, Thomas was diagnosed with severe autism at aged 3, Kylie has come to terms with this now and has adapted on how her brother is and how he behaves, at the moment he has a mental age of 3 he still loves the Teletubbies and Tweenies and only plays with certain toys, you need to just be understanding and understand that’s the way she is and its just not something that’s just going to go away, you don’t say how old your sister is or how severe she is, be there for her and help her when she needs helping, explain to your friends about your sister, one thing about Kylie she has always taken the time to tell people why Thomas screams a lot or still wears nappies and some do understand, but unless you face up to your sisters autism, no one will be none the wiser x