I understand your problem. Remember that both of you are in your teens & during adolescence people can be very arrogant, stubborn etc. So you must figure out the right way to deal with your sister. Do not order, command or boss over her to stop using bad words. This may create an adverse effect & can make your problem worse .
Instead befriend her. Talk well to her, get her very close to you by taking her out, buying her gifts, cooking her some fav food of her's etc . Get her to a condition where she really feels that she is the closest to you & she can do anything for you. Then you must cleverly ask her to stop using bad language and she will.
In the process dont worry, keep ignoring them cuz ur aim should be to befriend her & not make her stop by force.
I hope it helps 😛