My best friend of almost 13 years, she's like a sister to me. Well I don't know how to start this off. We really drifted apart. She's always lying to me. Every time we make dinner dates she always finds excuses not to go. But yet she makes time to come to town and sleep with MY brother, yes my brother. What best friend does that? I wish it didn't hurt me that she doesn't make an effort for our friendship but it does.. . HELP ME!

If your not comfortable in this relationship then end it. Allow it to disapate. Maybe later you can pick it up again but for now stop setting yourself up for disappointment. Also you need to talk to your brother about this friends with benefits thing. Hes asking for major trouble on this one.

She doesn't sound like a great friend to me. Good friends don't lie to each other. Actually that's why we have best friends, just so we can talk openly and honestly about everything.

Why not just cool it for a while?

I agree with the other posters, sounds like she isn't a very good friend, and is making you unhappy with both the lying and the relationship with your brother. Your brother should care that this bothers you, but in the end, people will do what makes them happy.

I would say cut your losses, and look for other friends. Maybe you can rekindle your friendship when she matures or is ready to be a better friend.