My in-laws and "bonus" in-laws are first name. (Husband's parents are divorced and remarried) My husband calls my parents by their first names. Niblings on husband's side call everyone "Grandma/Grandpa [First Name]", except husband's stepdad is Pappy.
I don't think this is weird. Husband's maternal grandparents were alive for over 20 years of our marriage. Grandma died four years ago, Grandpa just celebrated his 95th birrhday this past Monday. (And Grandpa is still fully mentally aware, God bless him) I call(ed) them Grandma and Grandpa.
I have a friend my age that calls her in-laws Mom & Dad, and it's just plain weird.