I can't tell if this is going to be a case of two people's in-laws trying to marry or if it's them acting like a big sister/little brother in law kinda thing.
Either way, it's like watching tiny versions of MC and Kubo interacting which is adorable
I wonder what's the age difference between Saki and Seita (around 4 years or so? maybe more.) It's a huge difference while they are children, pretty much unimportant when they are adults.
And Seita is adorable. Kubo-san better kick things into gear before their little siblings grow up and beat them to it. If I understand correctly, it becomes illegal then?