Im sorry your having to deal with your sisters mess..

Do you and your sister live together? Regardless if you do or dont you need to set boundaries for yourself.. Take your self out of her terrible relationship. She will most likely be upset with you , but so be it, you need to protect your own well being and maybe with you keeping away might make her reconsider her relationship.. One can be hopeful.

She is not going to change her co dependency until she is ready and obviously she is not there yet. Hopefully she will realize that he is not healthy for her and she deserves better.

I do hope she is being safe with birth control and precautions against pregnancy and STD's its a threat to her since he is seeing other people.. the last thing she needs is a baby or a diseasee with someone like him.

Just my opinion..
Take care