The single most useful thing a person can do for a trans, nonbinary, or gender expansive loved one is to keep showing up. Not perfectly. Just steadily.
Use the name and pronouns they ask for
Practice in private. Mistakes happen; the recovery is to correct yourself and keep going, not to apologize at length. Long apologies center your discomfort instead of their identity.
Educate yourself elsewhere
Do the reading, the listening, and the unlearning on your own time. Do not lean on your loved one to teach you the basics. They are exhausted by that question on a level you may not see.
Defend without performing
Correct family members in private. Do not turn the dinner table into a stage. Public performances of allyship can be more about the ally's image than the loved one's safety.
Give them space to be a whole person
Their gender is real and important and not the only thing about them. Ask about work, creative life, relationships, the small things. Not everything has to be a gender conversation.
Get your own support
Your own feelings, fears, and grief are real and deserve a place to land. Land them with a therapist, a peer support group for family members, or a trusted friend, not with the loved one themselves.