I can’t determine if there is actual sexual incest going on, but there is certainly a problem with boundaries. Normal growing up means developing a separate sense of “I”, separating from parents and becoming able to live independently. This generally happens gradually from the age of two through adolescence. By the time he is in his twenties, the “child” is ready to leave home and redefine his relationship with his parents — unless there are significant special needs or a significant mental illness.
From your description, it sounds like neither your cousin nor your aunt are doing that positive work. It’s hard to know where one leaves off and the other begins. I would be concerned that your aunt allows it because she is afraid of him except that it seems she is willingly cooperating with this unhealthy relationship.
I have to wonder where your uncle is in this. Why isn’t he insisting that his son grow up and be respectful of the aunt and uncle’s right to privacy?
They (aunt, uncle, cousin) are all adults. Although unfortunate, there is nothing illegal about the behaviors you describe. I suspect that there is very little you can do about it, except express your concern to the couple and perhaps to see if other adult members of the family are equally alarmed. A family intervention to encourage therapy might (only “might”) be helpful. Unfortunately, it may take something tragic happening to get people’s attention. It’s a shame. Your cousin’s future doesn’t look at all promising unless there are changes.
I wish you well.