My 11 year old son still undreses infont of me. He shares a room with his 5 yr old brother if I am in their room with his brother and he needs to change he just undresses in front of me. I not uncommon for me to see him naked on almost a daily basis. We are both comfortable with this situation, how ever my sister was visitng from away with her 5 yr old son, we were both getting our little ones ready for bed when my 11 year old walked in and began to undress.
I assume from reading this that your sister had issues with your son undressing in front of her. I can understand her concern to some extent, perhaps he should have waited untill she was gone or at least checked with you or her if it was ok to undress. However knowing kids especially boys he likely didn't think, after all it was his room and if it normal for him to undress with you in the room he just did it. I know my own son is perfectly comfortable undressing in front of me, I have been in the bathroom many times as he was getting in or out of the shower, and will often walk around in underwear. I do however encourage him to show some modesty if other people other than immediate family are around.
