Hi. I can't give you a definite yes or no on your question, but will share with you my experiences.
DS2 was 2.5 when he was DX with autism. The following month I found out I was unexpectedly pregnant. At the time ds2 was extreme hard work, and it was a difficult time for us all. Ds2 was non verbal, did not like any change to his routine, did not like Ouse, would not lave the house, it was very stressful to say the least.
When Ds3 was born, ds2 showed no interest in him, it was as if there was no baby. I couldn't leave ds3 on the floor as ds2 would just all over him, stand on him with out realising he was there. When ds3 became mobile, ds2 would lead him into a different room and lock him in there, when he was toddling he would be led into the bedroom or hall cupboard and shut in there. We think it was because ds2 was either scared of him or wanted to protect him.
Ds3 is now 4 and they are best mates. Ds2 has learnt a lot from ds3, there are definite benefits from having a younger sibling. They play together, ds3 loves role play games and initiates them with Ds2, who does try and play along with him.
If your DS was not autistic, would you have had more children?