Have read your updates.
You do need to tackle your husband. But I think a more effective way of doing it might be to bypass all the heartfelt chat and feeling your way and just assume that he is playing along, that he is enjoying it at some level, and just give him a straight out piece of your mind.
'You're acting like a fool, and it's getting worse. You needn't jump to your own defence, I'm not claiming that you're about to have an affair with her or something - God, if I thought that, we wouldn't be having this conversation - but frankly, I am getting sick of you showing yourself up in public, and by extension showing up me and the kids. Look, SisX is a stupid, childish flirt - always has been. She's getting worse too, I suppose it's sour grapes at still being single - but I'm getting sick of the fact that her current way of taking this out on me is to act like a dog on heat around my husband. I'm MORE sick, however, of the fact that my adult husband's response to this is to practically hump her leg back.
Next time you get the come-on treatment from her, I would be really, really pleased if you actually had the maturity to respond like a married father. It would be lovely to not have to field sympathetic glances from our relatives as they look at YOU, yes YOU - not her.
Our family expects her to act like a twat - yes, they do. You know that. I don't expect to have to be happy with them eventually pegging you as the same kind of sad, immature little loser.'
And to Sis?
Pick your moment. Rehearse. And at some point, when she's just done something else, just made a snide remark or inappropriate move, you get her alone, and smile, and say -
'Do you know, you are really beginning to strike me as an absolutely vile little bitch. There's only one thing I want to say. I don't know what reaction you want from me when you act like a dog on heat around my childrens' father, your own brother-in-law. All I can tell you is the reaction you will eventually get, and that'll be when I finally decide that you really aren't a good person to have around my children and my family. Is that the outcome you want? Think on it, and get a bit of fucking respect for your sister or get out of my face. Permanently.'