All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
As a husband, a man is indeed entitled to great rights. His rights as a husband should be given precedence over the rights of the wife's parents, let alone anyone else. Please refer to Fataawa 9623 and 85167.
So, if your wife shows greater care for her son more than she does for you to the extent that she neglects your due rights, then she is doing wrong, especially as her son is an adult and is able to take care of himself. You should advise her kindly and wisely and clarify the Islamic ruling for her. If she persistently maintains her blameworthy behavior, then she is considered a recalcitrant wife. Please refer to Fatwa 85402 about the wife's recalcitrance.
If you own the house where you are staying or you pay the rent for it, then you are entitled to refuse that your wife's son stay in your house. You should, or even must, refuse his stay if you have children from her and you fear his bad influence on them. In this case, the damage should be warded off according to scholars. The Prophet, sallallaahu ‘alayhi wa sallam, said: "There shall be neither harm nor reciprocal harm." [Ahmad]