How can I get my boyfriends sister to leave us alone?

[john]

There is a reason why he removed me as a facebook friend and why he has been ignoring my text messages. His sister is too nosy and she won't stop pestering him about me. When we first met she asked him if he wanted to marry me someday and he said yes (we were still friends then) but that not now in a few years if things go well with us. Now his sister won't leave him alone about it.

She has no clue we became more then friends but she keeps pestering him about me and if she finds out we are more then friends she will tell his parents and they don't know that he is bisexual. His dad especially would have a freak attack to find out me and him are boyfriends. How can I get his sister to back down? Ever since we became more then friends he has really stepped things down and I rarely get to see him at all cause of his sister being always all over him.

He said it's not that he does not love me it's just his sister is getting on his back and he wants to make it seem like he does not talk to me anymore so she wont tell his parents anything. So you could say we are secretly dating cause no one but us knows about us being boyfriends. I respect his privacy and space and I told him it was all fine about him backing away in the mean time but I am wondering how to get his sister to back up from being all on his business or find a way so that she can be cool with us as more then friends. Suggestions?