How can I help my family understand that being gay is not a disorder?

Resources. There are some resources available for familes who have gay, lesbian or bisexual children including pflag and some books on coming out to your family. Sometimes, inviting family members to a medical appointment with an affirming provider may provide a safe place to explore these issues more.
Hard to understand. It's almost impossible to "make" anyone "understand". Probably the best case you can make for gay not being a disorder is to live your life in such a way that they will have no reason to believe that being gay is a disorder. What we say about things has little to do with how we are judged as a person. It's how we conduct ourselves that tells people if we are "disordered" or not.