Truth in Love when Family is in Sin

{Truth in Love when Family is in Sin} My fiancé's sibling is transgender. I'm scared our future children will think it is a good thing and that it’s right. His family accepts this lifestyle unquestionably. I don't want to cut them out of our lives, but I don't know what else to do to protect our family. How can I be obedient to God and still be loving to his sibling and the rest of the family?