My sister's talk MAJOR sh*t about me and I can't figure out why!!?

I'm 20 years old, and I have 2 older sisters 22 and 25. They are BEST FRIENDS, to the extreme. They don't even call eachother "Sisters", they actually tell everyone they're best friends. I feel like the black sheep because they always leave me out of everything, and only talk to me when they need something from me. It's worse then I'm making it sound. But right now they're fighting because my 25 yr old sister is dating someone we dont like at all (druggie, no job, etc). So now they're only being nice to me because they're trying to make the other jealous, or to make the other feel like they're outnumbered now. Idk. But my sister (22) asked me to read one of her fb messages for a completely different reason, but when I went in her inbox i saw a message that was COMPLETELY trash talking me, COMPLETELY. They said things like i'm a slut, and how no one wants to be around me and the only reason they tolerate me is because I'm family. They even said other things were so heartless I couldn't stop crying. My own sisters. I am a very nice person, and I don't do anything to offend anyone, I just don't fit in with my family, and I feel even more isolated right now. What do i do?!