My annoying brother.?

I am an 11 year old boy going through puberty, y signs are very noticeable like i have loads of pubic hair. My mum insits i still have baths with my 3 year o;ld brother but the only problem is that whne im in the bath with me he always touches my and tells my mum i have a hairy winky.It is really annoying as i dont want him to tell her before i do. She really wants me to show her and i want to show her. How shall i show her?