Lets hear your two cents about this!?

So my friends is a closeted bisexual. His very best friend is very cute and is a closeted bisexual also. They hang out all the time and are completely masculine. But they mess around all the time. His sister is a year older than him and has a crush on his best friend. She told him and asked him to hook them up. My friend doesn't want to because he doesn't want to be ******* the guy who is ******* his sister. But he has a girlfriend. Its very confusing to me but basically my friend hasn't told his best friend that his sister likes him because he doesn't want them together. but his sister thinks he did. I think he should just tell his best friend that she's into him and let him pick but he doesn't want to do it. What type of advice would you give to him as to making the best decision?