May have overstepped my boundaries

Hi! I am seeking some advice on how to help my autistic adult niece get some friends and something to do.

My autistic niece is now 23 and lives with her mother and a younger brother who is 19.
Until the death of my sister's husband several years ago, I do not believe that they could ever fully accept that their daughter was autistic.
Only after his death did my sister actually come to accept that her daughter might have a disorder of some sort. 

My autistic niece had a meltdown after his passing. She got some therapy then and was put on Celexa. 

She had individual assistance to help her get through high school. Though, I don’t believe her education level is that of a high school graduate. I would place her mental level at about the age of 12-14. She is interested in everything Disney, Looney-Tunes, Batman, etc. and knows volumes about them. She also loves animals and is a veritable encyclopedia of knowledge about them, as well.

She gets along well with her mother and brother, but has no friends of her own, spending 8-10 hours a day in her room. She stays up all night in her room, getting up between noon and 2:00pm every day.

I live 5 hours away, but we spend time together sometimes on the phone and texting. She is very creative, and we spend a lot of time talking about stories she comes up with, including details of the many different characters which inhabit them and their relationships with each other.

She is very bright.

My sister is very difficult to speak with regarding her daughter's disorder and future plans. I believe she suffers from some extreme avoidance to all of this.

Last weekend, I offered to go with my sister to a place for young adults with ASD. We visited the place and upon leaving, my sister had a bit of a meltdown. Clearly, it had triggered something inside her. She was furious that I was meddling and attempting to run her over. She claimed that it was none of my business. Perhaps she is correct. 

Perhaps I have crossed a line. And, I will confess that I was angry in my attempt to get my sister to do something for my niece to get some friends and have something to do.

I may not have handled things well or correctly, and I hope I have explained things adequately. 

I am seeking some advice on how to proceed.

Thanks very much,
Stringer.