H.P.
I'm a stepmother and wonder how you came to be the go-between for them. Maybe the husband had difficulty communicating with her and you kinda came to the rescue, not realizing that it would be permanent. (New wives and stepmothers tend to want to "rescue".) Maybe you were seen as the obvious choice because you are so good at keeping up with these things...and communicating about the kids. Your husband (and their bio mother) came to depend on it.
Because you live with the children full time, you have taken on the role of full-time parent. This goes with the territory. I wonder if it's a problem for you now because they are at a kinda irritating age...and they are not your biological children...and your bio child is requiring more of your time and attention...and maybe you just have times when you wish you could just focus on your bio child and your husband and not so much on the steps. Consider this: If you were biological mother to all of these children, you would sometimes feel overwhelmed and want your husband (or others, for that matter) to lighten your load. Because they are not your biological children, you feel like you have an out by saying that you are not their mother.
If they were your bio children, you would want to get all the calls. Well, consider yourself lucky--both bio parents give you the credit for the role that you play as full-time mother, which is what you took on when you married him. From your post, it doesn't sound like you have extra work, but only that it's getting to you right now. Maybe you need to take a little break. It shouldn't be permanent, though. Your "a little about me" speaks volumes to the difference you make between their children and your child. You're gonna crash if you continue to move forward with that attitude. You might not even realize it. Those are all your children...and siblings to each other. You are by law a stepparent, but in your home you are parent to three. If necessary, get some stepparent couseling/guidance for perspective, before it gets out of hand and does damage to your family.