Misbehaving with her sister, what to do?

I have been going around with this girl for almost a year. We had at times minor quarrels but overall our relationship is very loving and very caring. Whenever we had a quarrel we sorted it out the next day itself. We have been sleeping together every night though we live in different homes (We stay in the same building). Tuesday night her sister and three of us had a drink. I got so drunk, and I could not believe myself, I happened to touch and caress her sister. I denied that I did intentionally, but she refused to believe me. I honestly dont have any feeling for her sister. I was feeling so guilty. She said she could not forgive me and she calls it quits. I am very depressed. I know she loves me a lot. Why should i cheat on her? I could not understand myself. What should I do to regain her love?