I don't think Karen married Hal "just to get away from her ex-husband" - she already knows that Hal is planning to steal David away, and it sounds like her ex gave him more leverage to win a custody battle against her; so she's trying to make the best of a bad situation. I do hate that she slowly (not suddenly) falls in love with him though. The guy's both a blackmailer and a rapist, and the only person he's apparently decent around/to is David. I also don't think she actually knew he wanted revenge - I think that was an inner monologue that Hal had; although to be fair there were many parts of this manga that were choppy and confusing, so I could be wrong.
In terms of the sister - I think it's just one of those deathbed confession things. Again, there were many parts of this manga that were just plain confusing, but it sounded like Kirsty and Hal were still working together even though they didn't get married. It never was very clear whether Kristy or Hal was lying about their relationship though… I think Kirsty was the one who didn't want to get married, but I don't know why. I guess all she really wanted was to have a baby for her sister Karen's sake, but I think she at least cared enough about Hal to choose him as the father.
I'm not happy with this. The late Twin said that the boys Father wouldn't marry her. So she had the boy and gave it to her sister. But from what I can tell is that the late sister didn't look like a large presents at all in the boys life so why was she with the child's father when she died? Why would she tell him when there was no point. The man clearly just wanted revenge from the start.
If you ask me the living sister was a complete fool. She married a man she didn't love or even want anything to do with just to get away from her ex-husband. She just suddenly falls in love with this guy knowing he wants revenge and yet she sleeps with him willingly and like and even bigger fool she doesn't try and stop any of it. This is sick.