YGirl - you took the words right off my fingertips - I was just going to say the same thing.
She know's she's not the only one, but she also doesn't know how significant my relationship is with him.
The other girl has a right to know what she is getting into and you deserve to be more than a secret or an insignificant person in his life. Waiting until they have been seeing each other for 6 months, I think, is just cruel.
You have a tough decision to make. Best of luck to you!