Unpopular opinion: I actually prefer she didn’t get a divorce before isekaing. It makes the idiot husband suffer. According to Japanese law, presumption of death due to disappearance takes 3 years for a divorce! (Unsure though, I am not a Japanese lawyer) Lets see if the girlfriend would last that long. It seems he can’t call his girlfriend until the divorce is settled and can’t move in with girlfriend before the divorce settles. Lets test if their bond is strong lolololol (〜 ̄△ ̄)〜
I predict the sister is the black magic the mages summoned with all the sacrifices. ヽ(`Д´)ノ