Not sure if I am BU or if it's okay to insist this with my mother, who is currently raging at me.
For a bit of context: DM is on her fourth husband, currently married to step father for about 7 years. They met and married long after I had moved abroad, so I had very little interaction with him. I moved back last year to have DS, who is now 7 months old.
I cannot stand my step father. He is a vile, arrogant man. I haven't been able to stand him any time I've met him, and especially not when I moved back and had regular contact. Things reached a breaking point around Christmas when he called me, to my face, a "selfish little b%#^h" for gifting them a trip abroad, all expenses paid, that was my mother's dream destination because he had apparently wanted to give that to her for her milestone birthday next month. I don't know if he would have, we didn't discuss it as it didn't even occur to me he would plan similar. He hadn't bought tickets or anything when I gifted it to them. After that, I've gone NC with him and refuse to be around him.
My DB and SIL, as well as my grandfather and aunt, and her husband's own two DSs also refuse to have contact with him. My mother frequently says she wants to get divorced but can't because of financial reasons, and thus will be sticking it out long term.
Anyway, to cut to the heart of the issue, DM was over today showing pictures of a weekend away to myself and infant DS. I said something to the effect of, "Oh, look at Grandma and 'Husband's Name' in their silly hats". DM corrected me to "Grandma and Grandpa". I said no, that's not going to be the case, besides my DF is already his grandpa. She promptly lost it, yelling at me that I have no right to be disrespectful and DS will be calling him grandpa. I said I have the right to decide what my child calls people, she said I didn't.
So AIBU? Willing to be told I am, but considering limiting her interactions with DS to solely at my home where he won't be around her husband and no need to address him as any particular name.
On a side note, DF is also very against her husband being called grandpa, as that's his special name, and DM insisted no other variations could be used, even though if I'm being honest, I'm not that open minded to any variation of a grandfatherly endearment.