I have a friend who has called her parents by their first names, probably since high school. I was reminded of her constantly this past weekend because Helen has taken to calling me Elaine with great force. It happened like this.
Typically, she would call for me "mom", "mommy", "ELAINE!". Because Helen has developed a habit of talking whenever Connor starts talking, and because I would like her voice to not drown out his, as a matter of course, I continue listening to Connor, respond to him, and then if I can understand what Helen wanted as well, I respond to her. It's a talent to listen to two people at once while simultaneously doing whatever I am tasked with, which could be driving! If I don't pick up on what Helen was saying, I ask her to repeat, and then I respond. Because Helen is a persistent booger, it usually means I am responding to her at the point she screams "ELAINE!", rather than the point she says "mom".
So, in her brain, it makes perfect sense that calling "ELAINE!" gets a quicker response than "mom", so now she just goes straight to that.
I have avoided squelching the behavior for three reasons.
1. It makes sense to me that we're on a first name basis. She knows me better than anyone else she knows and I know her better than anyone else knows her.
2. It's funny...especially when we're in public. I always imagine that people look at us and think "wow, that stepmom is the closest thing to a mom I've ever seen".
3. We'll be visiting Dick in a few days, and I know in his heart it annoys him to be called Dick, rather than Grandpa, but since it doesn't bother me that Helen calls him Dick, I've never bothered correcting it. It seems in this case, turnabout is fair play.