Did my cousin's older brother deserved to be disowned and disinherited?

I always thought my aunt and uncle were a great couple but it was all just appearances. Their two boys at the time were 7 and 3 years old.

It turns out my aunt had an affair long ago with her manager. My uncle discovered the infidelity through an old receipt. He did a paternity test on both boys. Only the younger child is his. The older one isn't.

He agreed to only stay for the sake of his younger child but as soon as the older becomes of legal age the truth will be revealed, he'll divorce her and he needs to find elsewhere to live.

He turned 18 last month. As promised, my uncle revealed him the truth, told him he had two weeks to find a place to leave, that he'll both disown and disinherit him because he's not his son and therefore not his responsibility, that only his mother is responsible for his college tuition. My uncle started the divorce process recently as well as joint custody for his bio child.

The boy is devastaded and feels both hurt and betrayed as he was the only father he ever knew and loved. Did he deserved that?

Did my cousin's older brother deserved to be disowned and disinherited?
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