Being trans* has a tendency to make one aware of all the gendered terms out there. “Boyfriend”, “Girlfriend”, “Brother”, “Sister”, “Aunt”, and “Uncle” leave little room for ambiguity, which can be frustrating for non binary identified individuals (and even, sometimes, for binary identified trans* people as well). Here are some options, some already mainstream, some invented.
*Instead of brother or sister, you could use “sibling” or “sib”
*For niece or nephew, “nibling” or “nephling” (both are used as a gender neutral term for a niece or nephew on urban dictionary) or “sibkid”.
*Instead of aunt or uncle, have your nibling come up with a nickname for you (for example, my niece calls me “T”, which was once short for “auntie” but now is perfectly gender neutral) or just invent something like “untiee” (a combination of “uncle” and “auntie”).
*Boyfriend or girlfriend can easily be replaced by “partner”, “significant other”, “SO”, “lover”, or “sweetheart”.
*Similarly, husband and wife can be replaced by the above words (particularly “partner”) as well as “spouse”
*For mom and dad there’s always “parent” or “parental unit” (as well as endless possibilities for made-up names, which we absolutely encourage)
I’m currently in the process of gathering gender neutral terms for the resources section of Practical Androgyny, some cute suggestions here :)