Need help with my girlfriend!?

I have been with my girlfriend for alomst 2 years and we have a great relationship..or so I thought. Last night I came home and caught her having with my mother and younger brother..I thought I was hallucinating. Unfortunately, I was not. The worst part is, when I walked in on them, they actually askedme to get undressed and join them. I love my girlfriend, but, my family has been there for me throughout everything and the betrayal I feel is beyond anything I have ever felt. I am at my wits end and feel all alone. Any advice would be appreciated.