My brother-in-law, who has been with my sister since probably 1996 or so might be transphobic. I'm not really sure what to consider him.
He told me that he would not accept me as male, pretty much. Told me I would always be his kids' "aunt" blah blah.
BUT the rest of my entire family was supportive so we all, including his wife, just completely ignored his dumbness. I still hang around his kids, they call me my name, male pronouns, etc. He still kind of ignores me due to the fact that he probably knows I hate him, but he calls me male pronouns and my name as well.
So, in short, I guess I'm trying to say is if the rest of your family is supportive then hopefully he'll either drop by the wayside or shape up.