It wasn't a kiss, dude. You slept with your sister's best friend. That doesn't happen by accident. You have to start kissing, go somewhere private, get naked, and THEN have sex. You could have stopped this. Don't even try to tell me you had no control because I know you're smarter than that.
Here's the thing: You can't take back what happened. You have to tell your girlfriend you cheated on her. If you don't, you don't have any respect for her. You cheated on her. This was an enormous breach in trust, but it happened. And not telling her what happened is amoral.
If you don't tell her, you don't deserve her forgiveness. You don't want to tell your girlfriend because you're afraid she'll leave you. You're not keeping it a secret to preserve her feelings! You're doing it because you're being selfish. Don't be that guy. Please.
You wouldn't have stayed up after everyone went to bed with this family "friend" and allowed this door to open if you had a solid, trusting relationship with your partner. None of it is even remotely appropriate. How would you feel if your girlfriend did this with her brother's best friend? Doesn't sound so great, does it?