I'm going to shout and nag to my girlfriend's step-sister (she's 5 years old) because she was trying so hard to kiss me in the mouth
Oh my, why would a 5 year old try to kiss you in that manner?
But in the end, the kiss was really cute, I think I will put this in my diary
Children kissing you on the mouth like this is quite inappropriate. You aren't blood relatives so such actions shouldn't be permitted. It can cause lots of confusion for everyone, and it's best to just try to teach her why it's wrong.
I can't believe you are saying this.
I know it's really shocking, I just want to let you know the truth of it all. She is a minor and can't consent or know what she is truly doing, so I believe it is your duty to guide her away from it and not encourage it. How about coming back to tell her what she did was wrong?