I've been with SO since ss3 was barely 1 and ss4 was 2. He has 50/50 custody although last he checked he has the boys more like 77% since BM seem to find excuses to not have them during her time.
Most days I think SO is too naive/idealistic on how stepparent/stepchildren relationships work. A few months ago as we talked about getting pregnant sometimes this year, SO brings it up that he wants the ssons to call me "mom". the ssons call BM "mama", SO wants them to call me a different word for mother so that we can always tell which one of us (me or BM) the kids are talking about. When I first met the boys, ss4(he was 2.5 at the time) called me "mama", I nipped it in the bud because it made me feel uncomfortable.

While I love and care about my skids, I told SO that I want my biokid to be the first to call me "mom". That turned into a fight because SO wants me to continue treating the boys like they're mine and told me that when we have a child, he doesn't want me to spoil our child to the point that the skids would be jaleous that I'm not their BM. I have told him that no matter how great I treat them, the fact remains: I'm not their mother! His reponse? "in this house you are the mother!" To his credit since before we even move in together, SO gave me the "authority" to correct and discipline the boys. Things are not always perfect but I do feel like overall we're a team and when ss4 gives me attitude, SO tells him to go and apologize to me.

ss3 is almost potty trained, although he still has "accidents". Yesterday he had one of those accidents and pooed in his underwear. Since we had guests and SO was entertaining them, it fell on me to clean ss3 up, gave him a quick bath and take care of the poopy underwear. In the middle of cleaning poop, it dawned on me! I am doing the work of a mother! When we have the boys, I cook, SO bathe them but I help with getting dressed, brushing teeth, bedtime story, lay down with them till they fall asleep, help with the laundry, change diapers, helped with potty training, comfort them, provide love and support, I've even taken off work when one of them is sick and have to stay home, when there's an event at the school during our time with them I would make snacks/bake something, arrange playdates, helped teach ss4 how to ride a bike, postpone trips because BM has something better to do and wants us to keep them longer. The list can go on. Yes I didn't give birth to them, but in the last year, I've spent more time with them than BM has,I spend my hard earned money on them, I've taken care of them, stay up at night when they're sick or when ss4 has nightmares and can't sleep. So why should she get all the credit! I deserve to be called mom and not by my first name!