I need advice

I want to do something like the picture below with my very first child. However...

My DO has two kids from a previous marriage and I'm more than happy to include them in on that DIY project. But I'm at a loss on putting Daddy, "Mommy", his name, her name and my sons name.

My SO and I aren't engaged, we aren't married and his kids call me by my name.... how can I make this work with me going to be a mom soon.. sounds so silly to type this but... i just don't know... I thought maybe I could just make one for my so, me and my son but I want to include my SO's kids. I have helped raise them for over 2 years and I love them.

I'm not sure what to do about these kind of things... (I also wanted to get family xmas stockings but how can I put dad mom on the stockings and then have the kids names, clearly I could out my name but I want the dad/mom ones, I'm going to be a mom and it's an important thing to me..)