Idk I think it's pretty obvious that they're going to lead up to Wahei and Hotaka not being related - they seem to have written a lot of similarities into the loneliness of their character's, and that's probably intentional for the purpose of later character growth and plot points. Just a guess, but based off of Hotaka's actions so far, and the fact that their mom ditched them so easily when she got married again, plus the gaps between all of their ages, it seems like her mother has abandoned her periodically depending on her romances, so she tries really hard to be useful and not in the way.
Plus, Hotaka's at least 16 and Wahei's 28, so he'd have been 11 when she was born. His dad was still around when he was a kid and fullscale disappeared when he was older. Based on his father's flippant nature, and the seemingly flippant nature of the girls' mother, it's also pretty unlikely that either of their characters would have maintained a consistent relationship with each other from Hotaka's birth to Chiina and Chiise's. But that's of course, just my own speculation. It might not be that deep, but it seems like the author is trying to place parallels between Hotaka and Wahei in a lot of ways.