1) You don't have to share with her when your parents are busy- You are choosing to share with her, we are all have choices in life, its about taking ownership of them. The choice you had was either get in the bath with her OR you stay in the bathroom with her while she bathes, you made the choice to hop in with her, accept and own it.
2) I don' see why you have to question if this is right or wrong here- You are both females who both have vaginas, its not like your seeing anything new, its not like she is seeing anything new here. Also, bathing or being naked together when it comes to siblings or family memebers is not done for sexual intentions, its done to conserve water, and/or to help someone bathe. It's perfectly fine you bathed with her, and at 2, a) needs full on supervison to make sure she doesn't slip in the bath or do something foolish like stick her head underwater b) she still needs guidance to make sure she is washing her body and not splashing around.
I have to question YOU here, why do you think there is an issue?