That's easy. Marry the son to the sister, encourage the boy to start a harem. This will automatically annoy the sister/wife who will complain to the father/father-in-law. The MC's cultivation is the same as the father-in-law's because they are the same person, so by the rules of the system he needs to overcome himself. End result a family more united than is necessary, the son with a harem, nobody needs to leave the house, a super strong MC, Sweet home Alabama soundtrack. And a daughter no one cares about after all, female characters disappear after marriage, a side effect of jade skin after the loss of chastity.