How to tell parents that my brother and I have fallen hopelessly in love? We even share an apartment?

I think this will be a hard thing for your parents to accept no matter how you break it to them. It's very different and I'm sure they aren't expecting anything like this, I don't think any parent would be. It's going to be a big scary step for you and your brother, but if you two really want to be together then I guess you'll have to let the world know eventually...especially your parents. There really isn't a "gentle" way to tell them. Just get together with them and explain the situation to them.