You don't mention if your DD has partner or not. If she does, then presumably they will be the birth partner but you will probably be called upon for babysitting and generally helping out after the birth and you will have the enjoyment of getting to know the new baby.
You must still, understandably, be feeling very hurt and raw about the break up. Women do not usually stay in hospital very long these days so your ex and his new partner would probably only be babysitting for a short while.
My experience of extended families is that is best for the children to have everybody on board who cares about them and for them to be, at least superficially, as amicable with each other as possible. I know this will be very hard for you in view of what happened you to through no fault of your own.
I wish you all the best for the future. You are obviously a very caring and sensitive person. Your daughter is very lucky to have you and your grandchildren, present and future, are lucky too.