Don't say anything. Really, don't. It's their business. You will bring nothing good on yourself if you get in between this.

There's a spanish proverb that pretty much says no one should stick their nose in-between siblings or a married couple, which could certainly be applied here. I mean really the only thing you can (or should) do here is talk to your sister, which it sounds like you already have. Maybe even take some action by putting space between you two to let her know how upset this makes you. Unfortunately it sounds like she doesn't care about the actions she's taking right now, but it'll come back to bite her in the ass in one way or another.