Engaged to the Unidentified is based on a 4koma SoL strip with a little "mystery" to it... and that one is right there in the title and the colorful OP so I don't consider that a spoiler. The gimmicks are that Kobeni becomes engaged due to a arrangement of her grandfather and that her fiance and his loli-sister are actually aliens.
It is pretty much about them becoming closer to each other over the course of the season. That alien thing is something no one really cares about. Basicly every storyline can be traced back to either Kobeni (the apparent holy angel and best wife you could ever have) being sick, worried or someone being jealous of her or Mashiro (the loli). Too bad the show isn't very funny because then I might have cared about anything in the show. The OP also starts out weirdly sexual for no good reason (and yes, the OVA is an Onsen episode).