Katie's lesbian given subtext, and figure she's also fancied Hermione given how she's one of the first to come to her defense and such. She just never said anything, because always figured Hermione was straight. Sometime after the war, Hermione is single, realizing how toxic it is with Ron, they meet up and things go from there? Usually have Katie as professional Quidditch player, but her parents also work in the ministry, so there's ample chance for them to meet.