Binetsu Kuukan - Vol. 2 Ch. 10.5

[MangaDex]

 

[sakurabonx3]

Honestly if they get together.. ._. father in law? Mother in law...? father? mother?.. step siblings sound a bit complicated... I guess it's how you see the relationship?... since you're not really blood related and especially when the parents get together so late... (I don't really know the circumstances because I'm not living in that situation :/)
I mean even in Naoya's case now, if he calls him father, it'd definitely sound more like a father in law connotation xD

 

[Creutzfeldt]

dad deciding his future

 

[DankyWanks]

The Father can't even imagine his step son calling him anything else but father-in-law, quite the foreshadowing, i hope.