Even if little bro acts behaved around our MC, we've all seen how deranged he is, considering he's the one that egged on Charlotte to almost kill the ML. I have no doubt that if Jeremy ever found out how MC was actually treating her "toy," he would see that as a sign of weakness and no longer act so kindly. In this house, the only way you can survive is to gain respect through others fearing your strength. MC probably had to beat him up a few times to get him to acknowledge her as a relative.