Ok, who owns this house that you and your finance live in? Because if this is "his" home and you have no real claim to possession of it, then you are a guest in this home as well.

And your finance, does he see how "rude" his sister is being? And he doesn't say anything to her? Does he see how his sister is upsetting "your" sense of boundaries?
Because you had better pay attention to how he allows "his" family to cross "your" boundaries. This is very telling of how he "respects you"overall.

Who actually "pays" for these groceries? If "you" are the one shelling out the money then stop buying groceries and have the F buy them.

If this is "your" home and the F and his sister are taking over then you have even more right to be angry.

So it depends on who is owning and funding for this "sister in law to be" crossing boundaries. It sounds like she is used to crossing the boundaries of others and your F accepts this behavior. If he is going to be your partner then you and him need to talk. It is important that you don't "train him" that you are just going to allow his family to "invade on you".

Open Eyes