Sister takes a bath with my daughter

My sister is very close to my daughter, she spoils her and loves her like crazy, which I’m grateful for. But, she’s always taking a shower or bath with my daughter. That is what I’m not comfortable with. I know that my daughter is like her daughter too so it’s “no big deal” but the idea does not sit well with me. I don’t know how to tell her this without looking like an idiot