Zero's manga / #sister's boyfriend(1)

Vol 1 chpt 7. Guy with shitty parents adopted by his sisters family. His sisters kinda a bitch, but plays innocent(she gets better later on). Anyways, hes introduced to her to be fiance. They become friends, and at one point, possinly hook up? He doesnt remeber anything. Then the dude dissapears, taking his sisters money. He was a marriage scammer. However he never goes for guys, so ukes trying to make sense of things and figure out the truth. Edit: wow tf. This best have a good ending. Damn.