I'm trying to find a manga that I read part of years ago, it was lewd but I don't think it was a hentai. It was about two young kids who I think were brother and sister(although they may not have been blood related) who both got genderswapped. They managed to hide it from everyone for a while until they started developing and decided to take each others places. Sometime after this I think one of the sister's friends finds out but I'm not 100% on this, it may be from a different series.

Two scenes I remember pretty clearly are the boy showing his sister how to masturbate and another where he avoids people in swim class so they don't notice his breasts.