I gave my sister a kiss on the cheek after I let my boyfriend nut in my mouth. Funny, huh?
If your sister doesn't know about it then it's funny, I agree.
I want to get back at her.
Well, you could do it a little more nicely. I just mean to take into consideration if it was done to you, and how you would like it. In other words it's fine for now but next time choose more wisely.
It's only a kiss and I love my sister.
If you love her then do things that are 100% more positive to really show it. There are just certain standards out there that should be considered. If you didn't know that before then you do now, it never hurts to learn more about anything.