Is It A Sin?

So my cousin and my boyfriend were thinking about being roommates and getting an apartment together. I was thinking about asking them could I move in with them and be a roommate as well but I'm unsure if that's a sin or not because you know how it's a sin to move in together before marriage and I just wanted to know is it a sin to move in as "roommates" if we're dating. I know I might seem dumb for asking but im just curious about this lol. And btw my cousin's a boy so no comments trying to say my boyfriend would cheat with my cousin😂😂.