stepdad smacking my butt

Ok so my baby sister will go up to people and smack peoples butt and she'll say "I got your booty but" so me and my stepdad play a long with and after wards my sister will be like that wasn't me so today my family and I are at my grandpas for a Christmas party and stepdad has came up to me and smack my Burt and I'm like hey why'd u do that? And he's like oh that wasn't me that was your sister I think he might just be playing along with the whole thing but I don't if he's doing bcz he thinks I have a nice but also tonight I'm going to text my mom and be like I don't like it when ppl just come up to me and smack by butt any thoughts on this?? I'm very confused