I used to get undressed in front of my older sister when I was a young child.
Why would you get undressed in front of your sibling?
She used to tell me that I was cute and I liked to show off.
It's not okay to undress in front of a sibling. You should tell someone if your sister was making inappropriate comments that could have been sexual in nature.
I don't remember it making me feel different, I didn't interpret it to have any sexual undertones.
It's quite odd for siblings to undress like that and you shouldn't think that was normal behavior. Be aware of your sister's actions and watch how she behaves in front of other children.