Coz the brother is an actual bum and the mom knows he sucks so bad that he cant be chased out of the house or something bad will happen to him, she trust MC more and tbh looking at how he is, she is right, he is independant unlike the other shit manchild.
BUT I don't agree with her in the slightest, the reason why that idiot grew up to be a shithead is because she enabled it, as shown right here where she choosed to shelter him and get MC out when the problem was him.

I just read the raws with a shit translation app so im confused. Why tf does our mc get kicked out and not the brother??? Like the mc is still in highschool at that point. The brother is a full ass adult if im not mistaken. Also mc looks great all dressed up. Hope the ml kisses the mc infront of the brother after the wedding