I'm not a step mother, and to be quite honest, I wouldn't get myself into a relationship with a man who has children because I really don't think I would want to take on his children. But then, I am very close to my ex husband's nephews whom I've known since they were 9 and 14 years old, so maybe I could be a step mother. Both nephews have lived with me for a few months at a time as young adults, and I am very much their auntie even though their uncle and I have been apart for 10 years now.
My DS has a step mother - she and my ex married about 4 years ago. She has no children of her own and as she's in her early 50s won't be having any. My DS, who has Asperger's, is now 15 and she get on well with him - I see her being loving towards him, genuinely caring for me, and I know from speaking with him that he is able to confide in her when he has problems and she gives him the same advice that I give him. She is an only child, and is very close to her parents who treat my DS, her DSS, as a grandchild. Overall, I am glad that she is in my DS's life, especially as she seems to be something of a calming influence on my ex, and acts as a buffer between my ex and our DS if their relationship is strained or difficult.
You really don't seem to going to have that kind of relationship with your step children, which is sad, actually, for all of you.
Another thing to consider is that the arrangements for residence and access might change. My DS used to stay with me during the week and his DF 3 weekends out of 4, sometimes less. I moved for work reasons over 2 years ago, and that would have meant my DS changing school. He was distraught about the prospect of starting over in a new place and new school and long story short, the residence arrangement flipped over. He now stays with his DF and DSM during the week, and with me 3 weekends out of 4. How would you feel if that were to happen?