His Brother Is Insane!

I'm in a really difficult situation, I've been with my boyfriend for three years, and we are completely and utterly in love. However, I am on the verge of leaving him because of his brother.

I don't know why but Keith (Calvin's brother) has got it into his head that we are meant to be together, he constantly hounds me with calls when he knows im not with Calvin and frequently comes round to my house at night. He has even told their Grandma that we have slept together!!!!!! THIS HAS NEVER HAPPENED!!!!!! AND NEVER WILL!!!!!!!!!!

About 6 months ago I had a miscarriage and it was very traumatic for me and Calvin, but about a week after, I was round visiting their Grandma when she confronted me about the baby being Keiths....this was so humiliating so now i have to avoid all family situations.
I tried to talk to Calvin about what has been going on but he got really defensive when i mentioned his brother might like me and completely flipped out, not believing me. It took weeks to get back to normal so I dont want to talk to him again.

The reason I've come on this is to make Keith see what he's doing is really bad and is destroying my relationship with someone i truly love.