Yeh ur definitely NOT overthinking it the author did it on purpose. Its mad wierd but that's not even the only that wierds me out like his interactions is cute with his brother but OML everyone likes her even the WOMEN like nothing wrong with lesbians but oml everyone wants a piece of the pie like I actually think the only person that doesn't like her romantically is her BROTHER. Isn't that fucking insane?!!
You think having a half brother would have held him back?? If you reread the chapter of her with him when the rumors about them spread, in the end of the chapter, he mentally admitted that's he doesn't mind the rumors and he's waiting for his dad to pass away from old age and the Fl to be alone with her son and end up needing to rely on mark (step son)
He would totally try to go for her if she gets vulnerable after her husband's death
Though I doubt she'll end up with anyone even if her husband pass away¯_(ツ)_/¯ she's that loyal to him

The comic is rlly good but the only thing that's a tiny bit annoying is the fact that literally everyone likes her. She's completely devoted to her husband and yet everyone around her has a crush on her even the women, even her STEP SON?? It's a bit annoying but I deal with it.