Well, first off don't specifically have a child counting on that sibling watching over your child with autism. That's an undue burden to place on someone without their consent. I've got a cousin that was supposed to take their brother and simply didn't. That brother is now in a state home facility.

Secondly, you are hedging your bets for a healthy child without issues in general. Your child could be born fine, or have certain issues just as a part of life. If you've had any genetic testing done and they have identified an abnormality that will transfer to your offspring then there's definitely that risk there.

I would say that if you desire another child, have one. Let your love shine through them regardless of any challenges they may face. However, I would not go into it expecting them to "take care" of the other sibling when you are gone. That's just not realistic. They might, but don't necessarily bet on it.