I am stepmum to my two beautiful daughters (6 and 8). I also have a son (7) from a previous relationship and my husband and I have just had our first child together (5 months).
My daughters don't see their birth mother, DD1 was abandoned at a year old and DD2 was removed at birth. DH has full residency of them both and we have been together since they were 1 and 2.
My biggest problems have been;
1, The knowledge that no matter what I/we do our DDs will, we expect, want to find their birth Mum and despite us knowing all the nasty stuff we know about the abuse and neglect she caused we will have to smile sweetly, support both DDs and just be ready to pick up the pieces.
2, The attitude of others when they discover you are only a step parent (both dh and I have experience of this) and then the utter contempt they hold you in when they find out that both DDs were subject to care proceedings. Step parent + social services = evil, nasty and untrustworthy to most people.
3, I am the only Mum they have ever known, they both call me Mummy, something they both chose to do and lead the way with. Other people are disgusted by this.
4, DH would say from his point of view as a step parent to my son that it is the competitive attitude of ds's dad and how he tries to belittle dh at every turn. Why can't he just bloody acknowledge that dh is the main carer for ds and is doing a bloody good job of it!
I have loads more, but apart from being able to wave a magic wand and just make all the children mine and DH's and no one elses, I wouldn't change a thing! 