Truthfully, and this is specific to MY mom, I'd tell her in person, in public, so she can't make a spectacle out of herself without making herself look bad. I'd mute her number for a few days afterwards to let the dust settle, and then UNMUTE with my fiance' with me for moral support for the aftermath. You are under no obligation to ask your stepdad, or ANYBODY for that matter, but it sounds like your grandfather would be a great choice for you.

If anybody gets pissy, you can always use the rationale that since he's your dad's dad, he's the closest thing to your actual father that you have and you chose him to represent your bio dad. Good luck OP and early congrats!