So MIL and SIL said today 
They are convinced his traumatic birth caused ds2's problems. He was breech and yes, it was horrendous, BUT we don't really know whether this caused the problem or whether he would have been like that anyway.
I had no way of knowing, and when I gave birth to ds2, I took the obstetrician's advice, and he said it was best to go ahead with a natural birth.
Now it seems that I am to blame for ds2's SN.
I didn't bother arguing with them. But it is getting me down.
Please come on and give me a virtual hug and pat on the back.