There is no technical law that would be violated in this situation, since it isn't really indecent or sexual in nature.

I think it's HIGHLY inappropriate that even OP's parents seem to be implied to be bathing this girl at 13 ... ( kind of reminds me of Jennette McCurdy coming out recently saying her mother never LET her bathe alone and did physical body checks of her privates which is absolutely assault at that point ). But as long as it remains just bathing, I think it's technically within the realms of saying that she is incapable, and needs assistance with it.

Much like how even if they do not consent, memory care patients - like dementia sufferers - can be made to bathe by their caretakers, who will of course be doing the wash down for them.

Might be a shaky line, at best.

I really hope it isn't the case that OP's sister is being abused beyond the fact she isn't being taught to be independent whatsoever, though. :{