Olive, the thing I am having difficulty in understanding is not that he and your sister do not get along (that is unfortunate but it happens sometimes) but that he wishes to isolate you from her if I'm reading correctly.

I guess if he chose to ignore your sister and not be around her that is one thing, but he is asking you not to be around her and by default not be around your parents too. That is a big ask IMO. You two plan of having children, that is a time when extended family can be a real source of support, but given his way you won't have that. I think he is asking you to make a pretty big sacrifice.

The questions I would be asking myself if I were you is whether I'd feel a few years down the line I may be resentful if I go along with this.