The plot's not that good but there's several good gifs to be had from it.
So I stumbled on this show by finding a modified video of the closing with characters from the creepy otaku game I've been spending all my time on lately and that made me curious enough to watch the series. I wasn't really disappointed, but it certainly wasn't much to write home about.
A mildly romantic, mildly comedic, mildly slice-of-life story about a girl who meets her prearranged fiance on her sixteenth birthday... and later finds out he and his precocious little sister aren't quite human. Exactly what they are is never explained, and the cast felt fairly recycled: a single male, big, quiet and protective, only rarely taking part in scenes, the secretly perverted student council president older sister, the extremely attractive, domestic heroine and the precocious little sister all felt like I'd seen them before. The hint of mystery of the brother and sister not being human is the only original part of the series, and it's not explored deeply.
Animation is fine, execution is competent but there's just not a whole lot here. I say it's mildly romantic: but it's definitely not a romcom. It's mildly comedic: cute and occasionally giggle-worth but never actually funny. It's mildly slice of life: but there is the vaguest thread of a plot to hold things together and advance... without changing or resolving much.
It's not a bad show, the characters have a few charming moments: once you know the very vaguely explained basis for what the male character's 'not-human' nature is he actually becomes fairly charming in his stoic way, and the kind of slow slumping in love the heroine does does is cute, and watching the littlest sister run in terror from the eldest sister trying to make her adorable gets a snerk out of me. But there's no substance to it, it is exactly what it looks like, no deeper readings or secondary interpretations or much to hold your attention. It's not good watching anime, but it's probably fine background anime.
I wouldn't really recommend Engaged to the Unidentified to anyone who was looking for anything more than something to fill the time. I wouldn't actively recommend against watching it, either, though, so if it sounds or looks like your thing, by all means, enjoy.