I am not a step parent; however, my son has a step mom and a step dad. I NEVER had a problem w/ his father or stepmother being there as his parent. My husband has been there before he was my husband and continues to be there now. If his father ever had a problem, he didn't voice it to me. We are his parents and as much as my husband never tried to replace his father was actually more of his dad on a regular consistent basis than his father.
You should continue to do exactly what you are...being there for your kids (both biological and step). Many single parents do have to put their job first (even when they don't want to) to keep their job and support thier kids...it isn't easy (been there too).
I think you should keep the communication open with your step kids mom. You already told her you aren't trying to take her place. Make sure she knows you are there to help with whatever the kids need and although you are not their mother, you can be another parental figure/loving adult they can count on. Tell her you want to be able to help her and their father get them where they need to be so they don't miss out and none of you have to do it alone. It is great for the kids to see everyone co-parent together and have a strong support system!