It was a deus ex machina suffering ending for the sake of having a suffering ending. She seriously could have just moved out, since that's what they make her do anyway. You know, like normal people do once they graduate highschool anyway?
But no, apparently, despite their memory erasing being established as being very limited, they reveal that they have been secretly brainwashing her little by little since she was a kid, (which is the asspull to end all asspulls but that's besides the point right now), in order to erase her memories of her entire life and replace them. Karin is effectively killed by the hands of her family, and despite Kenta going through all that shit to save Karin and prove he's trustworthy and everything, they reward him with with a mindwiped gf that thinks her parents died years ago, and tell him to live the rest of his life pretending he isn't living a lie. She's managed to live the past roughly 10 years or something like that on the outside kinda separated from her family, and now that she doesn't have her blood multiplying issue, and has several friends and a boyfriend on the outside, life and things should have only gotten easier. It's not like she will outlive her family or something. It's seriously just kicking her down as far as they could at the highest point of her and Kenta's life.
Seriously an ending like this makes Kuma Miko's ending look like Machi won the lottery. At least when Machi got mindbroken she was at pretty much the lowest point in her life and the future wasn't particularly looking like it was going to get better.