Calling it now. It just feels like Amano's style to not waste a foxy lady. That step-mom has most likely been with Tomo, no mark my words - she and Tomo have most definitely done things. (I'd be ok with that.)
And the thing we always knew arrived, that affair.
Lol the dad is one chill guy. Picks her cause she's a meek and hot woman, he daughter tells her she's lonely and she might cheat on him. "It's okay, it's something to be expected". Not a single fuck given.
As for our MC's train of thought in the first part, oh boy dem depressive thoughts right there.