My girlfriend's sister kissed me! Gf too angry! HelP! Pls!?

Woah woah.. the kiss lasted for a minute? Wtf were you doing for the past 59 seconds?! Go over to her house and give her an apologetic card with flowers and tell her that you didn't tell her cause she didn't want her to flip. If she doesn't want to see you, leave it in her room. You can't do much unless she communicates in return.