<human>: Can you succinctly summarize the text in 1-3 sentences?

My sister moved in with me a couple months ago. I don't mind, I'm single right now, needed a roommate and she's good company. My problem is that she can be quite mopey. The problem is she ruins most of her relationships by cheating or not caring. She says she's got an addiction to having affairs and the best thing she can do for any guy she likes is just not date anymore. 

She's 26 and has pretty much decided it'll be easier for her to live as an old maid forever. I don't know how to help her beyond just kind of listening when she wants to vent. In the past she did cheat on a friend of mine but they're on good terms now. She's a good person, addiction to affairs aside, kind, always fun to hang out with and always there for her friends as well as our siblings and parents.

She herself has admitted her addiction as she describes it is incredibly shitty, not just to herself but to the people she's hurt. She doesn't know what to do. I don't know what to do or say to help her. For what it's worth she's been attending therapy for this.

I know this may not be the best place to ask for advice considering what infidelity posts usually look like regarding the cheater. I can't imagine there's going to be much sympathy for someone who is addicted to cheating but if anyone wants to at least look past their initial horror/disgust for my sister that you'll probably feel, that would be nice.
<bot>: Big sis is addicted to having affairs, moved in with me and is very depressed about her behaviour and the people she's hurt