Does my bf's sister thinks she's his wife? Please help!!?

I've been dating my bf for a year and a half and recently we've gotten engaged. We plan to get married next year. The thing is, his parents died ( he's 45) but... he has an older sister ( single) who cant stand our relationship, she's "competing" with me all the time, he even supports her financially, she doesn't work, she has a credit card which he pays for, he also pays for her rent... and I can go on... I don't want to sound like the crazy jealous gf but... I'm getting fed up... because...he CAN'T EVEN tell her we r engaged!! she can't stand it when she sees a picture of us on his computer, the worst thing is that he's TOO afraid of telling her we r looking for an apartment that he hid the brochures they gave us at the real state agency!!! and i can just go on... So now... I'm getting really upset, it's like he has a wife and i'm the lover. That's how i feel. It's very difficult to talk about these issues because i don't want to trash his sister, but i don't know what to do. Should i tell him? I don't want to get married if this situation continues, because I'm afraid it won't stop until he does something about it. Am i right?