Chances are they were already mentally "there", just needed to hear that "straw that broke the camel's back". I mean...when my Dad went back to NJ for his mom's funeral, he told my Uncle I was Autistic( for whatever reason it was any of his business, I have no idea)...and both he and my Aunt suddenly referred to me as "severely crippled". What made it worse was, being that my dad is completely spineless, he temporarily disregarded any knowledge he already had about Autism, and just sided with them.
Ironically, my Dad is also the black sheep of his family, as his family is a bunch of pricks & he's adopted.
It's like no matter what I tell him about his family, he tells himself he's getting acceptance that he really isn't.
But getting back to my main point, dude I think the only one that was in denial the whole time....I hate to say...was you. You wanted to believe that your family was warm and accepting of you despite some quirks, but the deep down truth was they really weren't; obviously, they weren't going to say anything, but better you found out this way than otherwise.
And hey....as George Carlin would say "F--- 'em!" They don't deserve you in their lives if they're so horrible to you.
Just remember- Einstein was also Autistic, and he barely spent time engaging in discussion with people; he had good reason too, and he won the Nobel Prize for his efforts. We hear all the time about Einstein and his greatness; do we hear about the other scientists that were in his quarters? No, there's a reason for it; figure it out 