We gone through similar situation when my son was 3 and we were expecting baby. It was un-beleiveable how good he was with baby. I think autistic kids don't get jealous from other like normal kids. And baby would be too young to share toys or anything else so no issues here. Plus they see baby day in day out so after few days they get used to infact my son miss his little sister when he don't see her around. Siblings are best gift for them. Of course has to very cautious about not leaving them alone, you might never know what things could trigger misbehave.

I get nervous about getting pregnant as well. My son is 6 and he get's a little jealous when i'm around other kids giving them attention, and will try and yell and has tried hitting kids who are crying.... I do have a friend with a 2 year old and they have been hanging around more. He's getting used to be around a girl in her terrible two's. His ABA therapists are also working with him at school though and it has gotten a bit better. He is a bit more tolerant. I think with him it's more sensory overload with the crying.
We play around with him at home that we are "crying" as well and he comes over with a pouty face and tries to kiss and hug us. I'm hoping since it's a kid he will be seeing every single day of his life, that he'll learn to either help soothe the baby or find a way to tolerate it.
I do wish you luck though! I would talk to her ABA team and see what they can suggest. If you have small kids in the family or friends that have little kids, get them around each other. Or get her a little doll and explain to her that soon she'll have a little sibling that she will help take care of... make it a game. Just think of ways that you can make this a good and easy transition so when the new baby gets here, it wont be a shocker.

Hi Jennifer, she is only getting ABA through school

Does she have ABA therapy?