My Ex Step Dad Is No Longer a Father

My ex step dad doesn't return my calls or emales. The most i've got is a hall marked birthday card with his name signed at the bottom. My younger siblin was hurt by the split, but she's still close with him. It would pain me more than anything to know that something i said affected their relationship. How do i not resent my sibbling for her continued relationship with the man i thought was a father to both of us? Please help shine a light on where i go from here.