I began a relationship with my now husband when his son was only three months old. I had never wanted to date a man with children but I loved him so much that I instantly fell in love with his son as well.
I knew going into the relationship that his son would always be number one and I would be number two. I would have it no other way because I was always and still am my parents' first priority. As our relationship has grown, so has my love for his son. He is no my number one priority as well. He has always been included in all family events (my family as well as his father's).
He has always known who his mother is and who I am, those lines have never been blurred for him. He doesn't call me mum, he's had a pet name for me since he could talk and continues to call me that to this day. I use to introduce him as "my boyfriend's son" then as "my step-son" but now that his father and I have started discussing having children I've decided to start introducing him as my son so that he does not feel left out. I think that that is appropriate as I've been in his life for so long, his mum and I get along really well and are both secure in our roles in his life. I will not however even mention to him calling me mum. If when I do have other children he picks up on calling me mum then so be it. I will be very pleased. But it has to be his decision.
Being a step parent is a choice, being a step child is not. Once you make up your mind to be a part of a blended family prepare yourself for the road ahead. My road had not always been a bed of roses, blending out family took a lot of work and a lot of compromise. I know that one day my step son will probably drop the "you're not my mother" line on me and I know that I will be crushed but it's what I signed up for.
Whether you like it or not for as long as you're married to their parent, your step child(ren) will be a big part of your life. As the consenting adult it is your job to make sure those children never feel left out, or unwelcomed. It will only sour your and your spouses relationship and it will deeply impact the children's lives.
If you're not prepared to love a step child as your own or at least as your best little friend, DO NOT marry their parents.