As your DIL wants you there, and as she will need support through this time I would say go.
Remebering what I was like after giving birth (and I didnt have c-sections), I am still very grateful for all the help and support I received in the aftermath of delivery of both my girls.
Having said that, do not take any rudeness from your son, if he starts, tell him calmly you will not be spoken to in a rude manner, as his mother, you have yourself gone through exactly what his wife is now going through and for that alone you deserve a little respect at least in the way he speaks to you, and remind him, you are doing him a favour for being there, not the other way round.
Then walk away from him (go get coffee or something if he starts at you). Be calm, be firm and be sure in your mind the reason why you are there ie because your DIL wants you there for support and your grandaughter wants you there because she loves you and you are her family.
At some point your son will grow up and realise what he is doing, till then do not take any crap from him. Dont let him walk all over you, be firm in a very polite manner. He has already lost respect from his own in-laws for treating you badly in front of them.
Then just you remember your grandchildren will grow up and they will tell their dad exactly how it is. Hopefully though your son will come to his senses before his own children begin to intervene.
Congrats on the impending new arrival