Mom wants me to help with potty training

Hi, so I’m 14 turning 15 in July and I have a 2 year old sister. My mom has decided to start potty training her and she asked if I would taking her with me to go potty so she could watch and learn. I don’t know I feel about this. I’m older now and want my privacy when going to the bathroom, especially when I have my period. I used to change her diapers so I don’t mind helping her wipe or getting cleaned up after an accident. I just don’t want her in with me when going to the bathroom. How I can tell my mom without sounding mean?