Can anyone help with some advice?

I am a mother of 3 beautiful daughters.  My 2 oldest daughters are for my 1st husband and then me and my husband now have a daughter together.  My 2 oldest daughters are 13 and 10.  Their dad is part of their life but works away every other month.  When we divorced we had THE most amicable divorce of anyone we know.  There was no arguing, we used the same attorney, agreed on child support and custody arrangements.  I am the domicilary parent.  Our arrangement was 2 and 2, 2 days with me then 2 days with him, when he was in town.  However, as the girls got older they would ask to stay longer and I had no problem with that.  There were times when the girls would stay for a week or 2 and as long as the kids were happy, momma was happy.  A couple of years ago their dad remarried.  This have slowly started to change.  I no longer speak with their dad regarding them, i find myself having to talk to the step mom.  Slowly he has "handed over" the reigns to her.  I have repeatedly asked him to please keep our kids between he and I.  When i chose to remarry, my new husband and I had an extensive conversation regarding my girls.  He knew what he could and couldn't do as a step parent in my book. (example, disciplining with time outs is understandable, spanking is not ok)  I also explained to my new husband that i expected to handle all situations whith bio dad b/c that is how it should be.  Although my new husband is a GREAT step dad, he knows his boundaries and respects my relationship with their dad.  NOW the step  mom, I feel, is really overstepping some boundaries.  I have been very open to allowing her to be an intricut part of the girls lives including her on things that some biological mothers would NEVER DO!  But I felt it was important for my girls to see me and step mom get along and me and their dad have always done.  The relationship started going sour when step mom emailed to ask me why the girls did not go to see her when dad was offshore and that she would like to set up specific times for them to go when he is gone.  I honestly didn't respond to her.  I typed an email to their dad explaining that when he is gone that is MY time and when he is here that is HIS time and that is when should she spend time with the girls.  I have let them go periodaclly to spend the night b/c i know it is fun for the girls to get out of the house every now and then but set up a set visitation schedule with the step mom was not what i wanted.  Now granted I am very thankful that she wants to spend time with my kids but I feel like i was more than generous to let the girls go whild dad was gone and to ask for visitation while he is not here was just ridiculous.  (My husband would NEVER call their dad and say he wanted visitation while i was gone)  Several other things have happened and it's to the point that the step mom has almost become hostile with my oldest daughter.  In the last couple of weeks she has sent ugly text messages to my daughter and it is getting out of hand.  I will not talk to the step mom about it as of yet b/c i feel like their dad needs to "control" his wife, not me.  My relationship is with him not her, although we have been having to deal with her.  Last nights text messages got pretty bad (over the fact that my daughter was questioning if she wanted to go to her dads or not and he's not even home yet) and the step mom really got ugly with my daughter.  yes my first reaction was to go MOMMA CRAZY on her but i didn't.  I told my daughter that she really needed to talk to her dad and let him know how her step mom was making her feel.  She is old enough to relay to her dad her feelings and that i would be happy to help her along the way but that if I keep calling him and telling him she's hurt he will think it's me and not my daughter. (my daughter is literally scared of the step mom)  SO my daughter calls me while I am at work this morning to tell me that step mom was calling and said she was on her way over to my house to talk to my daughter.  Mind you, I am at work, my kids stay home alone b/c i only work a few hours a day.  The step mom did not call me to ask if she could go over, she did not call me ask if it was OK for her to go over, she just showed up and told my daughter to come outside. (yes i fussed at my daughter and told her she should not open the door for ANYONE unless i am home) she precedes to tell my daughter that she wants to smooth things over before her dad gets home and how sorry she is and on and on.  SO HERE IS MY ISSUE. I have already decided to talk to their dad again.  I have an issue with how she talks to my daughter.  Call it her personality, call it her loving my kids to the extent of treating them as her own, call it what ever you want but at the end of the day, i do not want any adult talking to my daughter the way she has talked to her. (i've also talked to my daughter about respecting her as an adult and dads wife)  And showing up at my house unannounced with out me there to be with my daughter is completely overstepping boundaries!  She has referred to my daughters as HER DAUGHTERS she has referred to herself as "mom" when out and about.  The best part is the we share the same name, so she often uses her name to get information regarding my kids. (some people don't even realize that my daughters have a REAL mom not a step mom b/c they assume she is the mom)  Maybe this was just a way to vent through this board but i am very hurt for my daughters and yes a little for myself.  i have worked double time to try to accomodate their requests and just try to see it from her prespective but it seems as though it doesn't matter what i do, IT'S NEVER ENOUGH.  I find my self feeling like i'm the step mom and i should answer to her?!  Do I do what I said and just talk to the dad?  Do we sit down all four of us (me, my husband, dad and his wife) and hash it out?  Do I just by pass dad and go straight to her?  Any advice you can share would be great!  I really want this to be a win win for everyone but not at the expense of my girls being hurt.  Thanks!