This is a very old fanfic. I would say 7-8 years old.

So it's starts with the group going to a cabin in the mountains. Kagome thinks they are her friend until they stumble onto 4 way tryst and find they kept her around for her grades. She runs away and stumbles upon Sesshoumaru's cabin. He has had an obsession with her but has never told her. In the end they get together and let her "friend" make her think she was lost in the mountains, possibly dead.

When they get home Sesshoumaru and Kagome stay together but Sango tries to tear them apart.

If anyone remembers this story, please help. I would greatly like to reread it.