J.F.
K.,
I'm not a step parent.
I'm about to marry my fiance, we have a 7 mnth old son together and I have a daughter(3) from a previous marriage, as well, I have a stepmom who married my father when I was 3.
Explain to the kids, before you get married, that you are NOT trying to replace their mother, but that you love them and their father and want to be a part of their lives. Tell them that you will treat them fairly and you'll not favor your youngest over them, just because they come from another mommy.
When it comes to discipline, you're gonna have a hard time. You may hear(may not be now, since they are so young but later on)"you're not my real mom so I don't have to listen to you". That's when you let your soon-to-be step in and let them know immeadiately that you are their step-mom and they need to respect you as much as they respect him. Just like you will have to do later in life with your little girl if and when she finds out that your soon-to-be isn't her biological father.
There's will be rocks along the road, but there will also be alot of happy times and memories that you can look back on in life. My step-mom and I never quite saw eye-to-eye when I was growing up, I felt my father deserved to be treated differently, and we had different opinions about what a wife and mother were supposed to be. I'm 22 now, and I think back on those days and realize that I did nothing but torture and try to ruin her relationship with my father, and may have almost succeeded many times, I feel bad about it now. I never wanted her to not be in my life, I was hurting and rebelious...ya know??
Anyways, good luck, and hunny if you ever want to talk about any of this, including the pain your ex has caused you, write me...mine did the exact same thing to me when I was pregnant with my little girl.
Much love