I'm convinced blondie is the stalker by the past chapters. So, the likely way this is going to end is by sister-in-law getting hurt by that reveal. Given she died in the first life it'll probably be violent or heavily coercive somehow. Even if it isn't, her personality wouldn't likely forgive deception.
If that's true then she's not going to get together with brother-in-law. She'll have lived through a betrayal that he prevented the first time around. If they do get together then it's going to be rocky or a long time later.