You poor thing, op. All this stress. I think mummytime sums it up well.
Don't give your power away - you will be dealing with his family for a long time. Decide what you want and do it. Make sure in your mind it is as reasonable and kind a decision as you can make, and then go ahead.
You initially wanted parents only, so that could be your starting point.
Then decide how you feel about dsis and dn, also try to imagine how you would feel looking back if they were/weren't invited, and come to a conclusion on that basis.
Your parents seem to be good at giving you space.
Your dh-to-be seems to be handling this well, and certainly a lot better than many would. He is on your side.
Again, initially he didn't want his dsis there. Assuming he still doesn't, that is sorted as a starting point.
As for his dm - well, if you feel the best decision is still to invite her, then she has a choice, doesn't she, to come without dsis or not come.
Don't put the wedding off. It's for you, your oh, and your children. Could you not still have a meal/celebration afterwards? If his dm came along, would she really do/say anything that can touch you, just married with your family around you? And if she does, well, for the future you and your dh could decide she has burned her bridges.