After my BIL lost his job my sister asked me to help them pay off his car loan to the tune of over 5000 pounds. For the record, I loathe my BIL quite openly, though I'm always polite to him, and the feeling is mutual and this was 100% his car, not hers. Besides, he couldn't be told that the money came from me to spare his feelings (mine weren't considered).
I suggested that they sell the car and buy a cheaper one but of course that didn't suit them so I ended up agreeing to sending a monthly contribution, only because otherwise my parents, who were already sending her the same amount to cover bills, would be guilted into doubling theirs. His side couldn't possibly be asked, apparently. I said that I would pretend that this money was for my niece and nephew, which it obviously wasn't, and set up a monthly transfer so I wouldn't have to think too much about it.
After over two years of this and when I knew that the car would soon be paid off I had the pleasure of being informed about what she would be doing with this money once the debt was settled. No thanks, no next month is the last installment, no asking whether I wouldn't mind, just being told that she would get a cleaner and I don't remember what else. With my money, which was obviously not mine anymore, and this in spite of having promised to reimburse me and my parents (I never believed her and said so to my parents, who thought I was mad).
But it gets better. I told my mother, who thought that I must have misunderstood - no, mum, it was crystal clear - and spoke to her. Apparently she was told that I would now need the money for my baby (I guess that now I need an "excuse" to keep my own hard earned money and I mean hard-earned quite literally, I'm self-employed). The next I heard was the very long, very shouty angry rant I got from her at about 7-8 months pregnant when she called me all names under the sun and then some, all of this in front of our parents, for daring to mention this money in passing. Not asking for it back, not calling her a user or even calling her out on her attitude, just mentioning it.
Our relationship wasn't the best by then. Now I only talk to her when my mother is present because she can't accept that we don't get on and blames herself, therefore I pretend, her not so much. For the record, she's my older sister and I was barely earning when I started sending her money as I had been laid off and was starting to work as a freelancer. The first months' contributions came out of my savings. None of that means anything to her.
You don't really have a relationship to salvage. Even if you did before this situation, it's now well and truly dead because your sister resents you for your generosity and also for withdrawing it. Just accept it, ignore her and move on.