There's two ways to go about it though
1) You come away from this so unable to trust anyone you won't even hear them out
2) You do hear them out because you treat them the way you wish you had been treated when you were accused of wrongdoing.
It seems like the mc is going for the first option...
Not even gonna hear her out huh? For a guy who just went through a whole ordeal surrounding false accusations he sure is quick to believe some dude's words over his own sister's.
It depends entirely on when he and their mom became aware of her. There is still a potential scenario where they were aware of her long before they showed up to adopt her, taking an interest only when the mom wanted to impose her ambitions on her.
It is weird that only Yotsuba was given up for...