I am a stepmom too and I love my stepsons dearly, but yeah, sometimes it really bites. They are now 16 and 17, and I've been a part of their lives the past 10 years. Their dad and I got married 6 years ago and now have a 3 year old girl of our own. Their parents separated when they were 4 and 5 so I had nothing to do with that. The boys are wonderful and their mom has been great for the most part, but it had not been a bed of roses either. I have found myself get irritated simply because she handles things way different sometimes than I would. She's not been as strict with them as maybe I would have been but I've learned that how they are disciplined by her and their dad isn't really any of my business and I'm better off just butting out. Like when they got caught last year smoking pot and Mom decided to ground them for a month, but then it ended up only being 2 weeks. Now the oldest one is a senior in high school and talks like he wants to go to college but has not taken any of the necessary steps (like taking the SAT, getting applications together, etc.). This worries me, but again, it's up to his parents to deal with him, not me. Anytime I've tried to get more involved, beyond just being "fun and games" stepmom, I've gotten an earful from their mom about how I am not their mom and it's not my place to discipline them and she will handle it. It does not help that often they would go to their mom with complaints about me and she would get all upset before hearing my side of the story or considering that maybe I did have a valid point, despite not being a mother myself at that time. I've learned to pick and choose my battles and when one of them gets out of line, I just tell their dad about it and let him handle it.
I think it's probably harder to be a stepmom to girls rather than boys. I think sometimes you just do the best you can, make the decisions you make, and realize that not everyone is going to happy about it. I would let their mom and dad handle the discipline and if they want to pick fights with you, just let them know that you will not tolerate any disrespect and you refuse to argue with them. If they have an issue, they can take it up with their dad, and hopefully he will back you up.
p.s. I would not get hung up on them calling you Mom. They have a Mom, they should be able to call you what they feel comfortable calling you. My stepsons have always just called me by my first name, same as many of our close adult friends.
p.s.s. I just read your "so what happened" entry, and saw that you are 27. And your oldest stepdaughter is 13. Which is only a 14 year difference. Maybe that could be a reason she's having trouble accepting you?