They're both trash and personally I would find my relationship with both parties irredeemable.

Your sister knows damn well what she's doing. Even IF (and that's a big if) she's really not into your husband, she encourages his behavior.

She is liking the attention he is giving her. Not once has she tried stopping it, before or after you made her aware that it's disrespectful.

For your husband... I don't really need to say much. He's a cheating bastard.

Both of them lied, are still lying and gaslighting you into thinking you're wrong and taking it too seriously. I would not be surprised if something physical did happen and they're trickle-truthing you about it.

It's time to call a lawyer.