There's nothing wrong with that. Me and my sister bathed together until we were about 5 and 6 (we had a smaller age difference, but whatever). It's not like bathing with her sister is going to mess her up or scar her for life. Forming close bonds like that is a good thing.
I know! Why cant those stupid ignorant people get it through their thick heads? Sexual orientation is predetermined before birth. No matter how many times I say it there all like "LALALA I CANT HEAR YOU!".