Men and women can’t be friends, and it was a mistake for you to suggest that your boyfriend and your sister share an apartment as roommates. 😕 They wouldn’t be the first couple who met as platonic roommates and became more than that — although it isn’t clear how much more. But that’s over and done with — just learn from that mistake. 😉
Now that you and your boyfriend are back together again after a break up, you’ve got competition — and it’s your sister. And it sounds like she has a history and a pattern of competing with you for men, so rather than try and thwart her, which will just cause her to become more covert, and probably make the challenge more interesting for her, don’t put any attention or energy onto her, and put the energy and attention you have into the relationship with your boyfriend. The reality is that the world is full of competition, and whether it’s your sister, a secretary, his accountant or a stranger on a bus, you will have competition. Dial up your A game, and make the relationship one he doesn’t want to leave. If you try and give him ultimatums, or banish her from contacting him, you’ll lose out and spend a lot of energy doing so. Instead, focus on what’s good between the two of you and be the girlfriend he doesn’t want to upset or risk losing. 🙂
I hope that helps. Let me know if you have any more questions.