My 21 yo niece is visiting

My 8 year old Son was all excited to see his older cousin and fell asleep in the guest bedroom talking with her. She's my wife's older sister's daughter. I went to get my son and put him to bed and there she was in her slip on. I had never noticed her like that before.

I picked up my son, and with him in my arm I leaned in to kiss my niece good night on the cheek of course. She's a good foot shorter than me, so it ended up being a little awkward as she leaned in and I ended up planting a kiss half on her lips. I'm not sure why, it just sent a current through my spine and I didn't know what to feel now. I was embarrassed but got such a high from it, especially as she seemed to not mind at all and walked with me talking about random catch up stuff as I went to put my son to bed. When she was ready to go back to bed in 5 minutes after our chitchat, she leaned in again for a kiss and gave me a half lipper and smiled and left. Now I'm next to my wife and gonna wake her up and f*** her thinking of the 21year old neice. I can't wait for the Morning.