It's not weird or forbidden because she is your sister and you're female. It would be wrong for teenage males to share a bath with a younger sibling, but it's OK for teenage females. Males represent a sexual threat, but as a female you can be trusted not to do anything untoward, and nobody will be suspicious of you. It's good for her to understand the changes that will one day happen to her body.
The only question is whether you enjoy this arrangement or not. It sounds like you will have to bathe your sister anyway, though many parents can bathe toddlers without getting into the bath themselves. If you would rather take a bath by yourself then bathe her beforehand wearing old clothes (as she'll probably splash you) then take your own bath. But if you don't mind bathing with her, then nobody else will mind either, and it will save time.
Start teaching her how to take her own bath: she's too young to bathe alone yet, but in a couple of years she will have picked it up.