While I do think this new relationship is a little weird, I'm not super bothered by the whole in-law thing like some of the other commenters. My issue is this: the story is about our main couple, theres way more the author can do to develop them, so why waste space on a couple that isn't the focus and doesn't add to the story when you could further develops the MCs? This second couple seems 100% unnecessary, and honestly fairly detrimental to the manga as a whole