What is the ruling on dealing with the stepdaughter who committed a sin with her cousin and then married him, and is it obligatory to boycott them or to continue dealing with them as before?

The daughter of your stepmother, if she is not your father's daughter, is a stranger to you (ajnabiyyah), so it is not permissible for you to look at her nor to be in seclusion with her. As for your paternal uncle and your paternal cousin, do not abandon them. Invite your paternal cousin with kind words and a gentle approach. Abandonment (hajr) was legislated to deter the one who commits a sin, but if it leads to the empowerment of human and jinn devils over him, then it contradicts the intent of the Sharia.