Hi Billijo,
I just had to respond to your delimma. I am on the "other" side of your problem. I am a "step-mom", but the problems came from the Mom. I never put myself into her position with the kids. They were 10 & 12 when my husband and I got married. I also brought a 3 year old into the mix. I was often accused of trying to be their "mother" and in my own defense, we made it clear from the get go that they had a mom and that, even though I was now in a parental position, I was NOT the mom. I loved them as a mother would and looked out for their well being, as a mother would, but when it came down to it, I always reverted back to the fact that I wasn't the mom. They have always called me by my first name. That has never been questioned. Without getting into a long drawn out story about my life and woes, suffice it to say that these kids have known and know that I am a constant in their life, and have often come to me for advice (moreso "motherly" advice) because I can step back and help them look at the big picture. The kids are now 24, 22 and 15 and with the exception of a few horrendous situations, I would not change a thing. I love them and I know that they love me. This probably sounds a lot like the ramblings of a crazy person, but keep the faith, talk to your ex and if that doesn't help, talk to his girlfriend (if that is possible) and let her know your feelings, she may be being told other things from your ex and feels that her being called "the other mom" is acceptable. I would definately talk to your girls, albeit, they are kinda young and just gently re-inforce your position. I don't know if it is a good thing or a bad thing, but I would strongly urge them to only call this "girlfriend" by her first name. Just be careful that you don't put them in the middle of something and they wind up feeling bad or getting hurt or reprimanded.
I am sorry this is kinda long, but as I said, this just skims the surface of what I would have liked to say. I hope this helps.
K.