Kinda seems like a skill issue tbh. You've really already reincarnated a bunch of times and you can't solve this?
Her sister seems to love her. She could use that. As soon as the loop restarts, she should pull her sister aside and confide in her about her troubles with her fiance. The sister and fiance have only just met, so there's still the opportunity to define how the sister sees him. Tell her about how he has ignored you, flirted with other women, told you to suck it up while you're feeling unwell (the sister, being chronically sick, should be sensitive to this), and of course you should lie and exaggerate his misdeeds. Beg your sister not to tell him what you've told her, then continue to have private meetings with her to ask her advice on what to do about your horrible, abusive fiance. Ideally she should be talking about his misdeeds with you more than she's actually meeting him face to face, that way his kindness to her will seem like a lie or a deliberate cruelty towards you. Your sister should be thinking, "He's flirting with me right in front of her? What a bastard! How could you be so cruel to my beloved sister!" Of course, you should also make sure to do more nice things for your sister than your fiance does, so that she has a more favorable impression of you than him. Dote on her, give her gifts, care for her when she is sick, etc.
These protagonists always act so passive and helpless. A REAL villainess would manipulate everyone involved and win TOTAL VICTORY.