You're looking for Till Death Do Us Part. Possibly also Temptation of the Demon but nobody's synopsized that for the wiki yet and I forget what happened in that ep so I can't confirm.

*shrugs* Four adults all playing parental roles to four children held in common, is one of the key points that's got me thinking she would be writing a sedoretu if she were not heterosexist and if she knows what a sedoretu is.