My girlfriend's sister kissed me?

luisjr
I have been with my girl almost 7 months and our relationship is great despite a little age difference. I'm 25 and she is 19. I love her very much and I mean the world to her. Well, the other night I was at her church and at church everyone greets with cheek air kisses. When I was going to greet her younger sister (shes 17) that way she turned her face toward mine gave me a kiss right on the lips. I was standing there shocked and she said "oops sorry" with a smile and walked away. A little later my girl came out and I acted like nothing happened. What should I do? Should I tell my girl what happened?
My girlfriend's sister kissed me?
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