Should I obey my father who swore to disown me if I don't attend the family Sham el-Nessim gathering?

It is not permissible to celebrate Sham El-Nessim, nor is it permissible to obey one's father by gathering with the intention of celebrating it, even if he threatens to disown; because obedience to parents is only in what is good and permissible. You should explain the ruling on celebrating to your father, and that your abstention from answering his call is due to the impermissibility of the celebration, while also showing him the words of the scholars, and strive to be dutiful and benevolent towards your father.