He said "nee-sama" so there's a chance he's a much younger sibling since a lot of families sometimes have wide age gaps between siblings but there's a good chance that I'm wrong
That us true and they are both aliens, at least from how he talked he sounds like he's a full breed alien at least. Though he doesn't have the same otherworldly jelly look like Shio's mom. I just the want Na-chan to end up with another adult male, especially after that shitty doctor.
Sometimes "nee-san" can be used for older women who are still young, as in, she's older then him but not old enough to be an auntie
I was wondering with the last chapter if it would end but that's not the case~ I'm glad to see Na-chan's story is expanding and it seems he's going to get his own alien boyfriend lol. But if that's Shio's uncle wouldn't that mean he's much older? He also looks half human considering if his sister is the mom, well her hair looks like when they splatter. Would honestly love for them to turn this into an anime later on