I have this experience with a child with ASD who is also pretty severe and who was older, 6 years old, when we had DD
I worried about all the same things as you did. I was pretty sure that I would not be able to leave DS2 alone with DD as he was unpredictable and occasionally aggressive.
My DH brought him to hospital when she was born. DS2 had not really understood the whole pregnant thing - we had tried to explain as had his school but we were not sure if he understood.
Atthe hospital he saw her but was more interested in me. Only when we went to leave did I realise how much he understood about it. As we were collecting my things he became very anxious. He managed eventually to make us understand that he was 'wowwied' that we were not taking 'Baby Madeleine' home.
He loves her. She is 7 now. She is fantastic and has started helping him with only a slight inclination to being bossy. he loves to watch her playing and will play very simple chasing stuff etc whichthey both love
When she was small it was hard to explain to her that they were treated differently - she could do things he could not and vice versa but she gets it now.
She goes to a girls school but still insists that DS2 joins in with her birthday party which he does ( he has been a fairy, a pirate and a witch ), he gets concerned when she is not about and they hold hands when we go out.
You HAVE to work extra hard at it, we have to make sure they both feel that they get special time and treats oftheir own, but DS2 loves her around and DD has learnt a great deal about compassion and sharing and love.
I suspect that at some stage she will go through stages of being embaressed or disintetested etc ( we have an older son and he did all of these things at times) but like DS1 I figure it will even out in the end.
It was a great experience for all of us. DS2 tries harder to play and be with his sister and has skills he would not were it not for her. DD is very wise and very sweet for a little 7 years old.
Its great