K.I.
M.,
OK, I am absolutely not trying to be critical here, but have you ever uttered the phrase, "If you girls are having a problem, you can come to me."? Telling a teenager that you don't want to hear about their difficulties with step-mom and that Dad has to be their point of contact is assuring them that you actually will NOT help them deal with their troubles. Teenagers already feel like nobody ever listens to them. How many teenage girls actually want to talk to their Dads about any problem?
It is pretty much a given that kids are not going to like their step-mom, if only because she is NOT Mom and takes Dad's attention. It is also a given that unless she is unusually loving, the step-mom is not going to be as kind and loving to "his" kids as she will be to her own. (We've all seen it, so all you really nice step-moms out there, do not take offense. There is actually a reason why all the fairy tales deal with the evil stepmother.)
Now, of course, there is venting, in which people complain about situations they do not like to get rid of the negative feelings. There is mindless whining in which people just feel sorry for themselves, which does no good. And then there are productive conversations in which your daughters can express their complaints to you, and you assist them in determining what is the true problem, and positive ways they can discuss it with their Dad & step-mom to arrive at a mutually beneficial solution.
Truly, you are their Mother, and even when they are not in your household it is still your duty to protect them. Many people feel threatened by vegetarians, criticized because they have different eating habits, or inadequate because they just don't know how to prepare a properly balanced meat-free meal. This conflict may have nothing to do with your daughter's habits, but more about her attitude. Please reassure your girls that you will help them solve their difficulties. You can even help them write a list of ideas, complaints, or discussion points with possible solutions.
Best wishes,
K.