Hi,
I lived in shared houses for years before I bought a house with my husband.
I think that in shared houses there can often be a sort of pecking order, that is unrelated to legal stuff, and is just down to the force of personality of the people in the house.
It sounds as though your housemate feels she is higher in the pecking order because the house belongs to a relative and because she was there first.
From a practical standpoint, it's a little more complicated because she can't afford the house without you. You could move out to another house where your boyfriend is welcome, but that would be a hassle with all sorts of unknowns.
I think what it comes down to is that you want your boyfriend to stay over and she doesn't. She can't manage without your rents, and you don't want to have to move out.
I think you might need to have a chat with her about what it is that you both want and how badly you want it.
Either she will capitulate and say your boyfriend can stay, or she might refuse, in which case you say that it's not worth the hassle of sharing with her.
It's a very delicate discussion and not easy to do when you've already tried and been knocked back.
Sorry this isn't super helpful. I think you just need to try to talk to her again. If you can't get sense into her, then you think about moving elsewhere, or getting your own place.
FWIW, I had this situation once and what I did was to find a large empty rented house, take the lease myself, and then find three others to move in with me. That moved me up the pecking order a bit and then I could live the way I wanted to. A little while later we got a new housemate who was really difficult to live with and things got harder again.
It's a bizarre thing, but that just seems to be how it is.