I don't know if this justifies the Sarah personality change but as far as Christian and Sarah, I think it has less to do with her and more to do with the MC's relationship with Warren's mafia. That was one of the things that was a bit weak in the original game, which was the exposition and explanation of the MC finding out that his dad and "uncle" are in the mob. This update makes it MUCH more clear that there's something very shady going on. And given that Jack is Warren's right-hand man, and thus have a close relationship, it would only make sense that Christian and MC (and Sarah) grew up together. This was something that was practically glossed over or ignored in the previous version. And given Christian's personality (which hasn't changed) and his hatred of MC, it would only make sense for him to go after Sarah to screw with the MC. And from Sarah's perspective, she has no reason to rebuke Christian because OBVIOUSLY he's been nothing but nice to her the whole time, and (thanks to her unpopular personality change), she no longer has a brother complex. At least not overtly.