He was actually not really worried. They had a clash and he was still angry at her. When the mother asked for his help he didn't want to search for her. It was the kid that pushed him to search for her. They haven't made up.
His sister wasn't much better than these 2. All through his youth she was constantly preventing him from getting a normal relationship with anybody, the moment she realized she wanted her brother to herself she instead sought solace on a random dude that "resembled" his brother. He rightfully fucked out of that household but the damage was already done.
This whole arc has been about how his actual family is just as broken and how the wife entangled the sister with sweet words so that when she eventually comes out with a reason why her daughter is also pregnant the sister would side with her. It is carefully built, brick by brick. And on the go, because there's no way she knew all of this information, she just pieced everything together in a few days.