Hi! I believe I can be of help with this issue. First, let me say that the hurt you feel comes through very acutely. And you are clearly an unusual stepmother. That you want to have a good relationship with your stepkids is really quite refreshing. But it is clear that you are hitting a brick wall.
And this is actually the key to my answer to you that you need to consider and think about. I am concerned that you not keep hitting your head against that wall. What do I mean?
Is their mother behind their actions. It's possible that she's consciously behind their standoffish behavior but it sounds as if she doesn't need to do anything consciously. Jess has already internalized the competition between you and your husband on one side and her mom on the other. The mother doesn't even have to say anything at this point, I'm sure. They are all so used to who are the good guys and who are the bad guys that they just live it without having to speak about it. So this is what you're up against.
And this is what I mean by hitting your head against the wall over and over. You keep trying to make peace gestures that are very sincere. But Jess isn't accepting and it's clear from her excuses that it would upset the balance in her life if she would move closer to you and her dad. That balance is having to deal with her mom. Her mom has NPD. With that in the mix, to make ANY gestures to you could cost Jess emotionally and it isn't worth it to her.
So those are the two stumbling blocks: mom and the kids have already cast you and dad in a role and second, if you would be treated not in that role, then the whole balance with mom would be upset.
So therefore, I recommend for your sake that you just accept it the way it is. Why make Jess' life more miserable than her mom already makes it? Be as nice as you are and accept that Jess isn't going to be able to reciprocate either because SHE's not up to it or because she's scared of setting off her mom. Either way, let it be.
I wish you the very best!
You know, you are of course right, we "cannot get inside their heads, or their mother's for that matter." But I can tell you from my experience what the most likely scenario is for Jess. You are sensitive enough to other people's feelings to identify if it might be accurate in Jess' situation:
Living with a narcissist is not fun. It is like learning how to make a life for yourself in the domain of a fire breathing dragon. You never know when the dragon is going to turn on her blow torch and start incinerating. But you can predict some things. So you order your life around what's worked in the past to let you have some autonomy but without setting off the dragon. Then the situation changes as it does every so often. Let's say two years ago. For whatever reason. Maybe the dragon got wind of some loyalty being diverted from her. Maybe she went through a menopausal change. Whatever it was, the wind changed in her domain. Jess had to react. Well, the way that's always worked is to appease the dragon. So, turn up the volume on how dad and his new wife are the enemy. The dragon calms down, the blow torch gets turned away from Jess. Jess can breathe a little easier. So she lost the opportunity to get to know dad and his wife and her brother. But the heat is off her back. And that counts for a lot.
In my practice, that's the leading scenario. I thought of it right away when you asked what could you wrap your head around that would explain it.
I want to share two books with you to give you an idea of what it's like for Jess with her mother. They're not specifically about mother/daughter relationships, but again, you are astute enough to see the points these authors are making that would apply to her mom:
1. The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship by Eleanor Payson. This is a great book that will help you with the lack of self-esteem that living with a narcissist or being close with a narcissist will do to you. Amazon: http://www.amazon.com/Wizard-Oz-Othim-Narcissists-Relationship/dp/0972072837/ref=sr_1_2?ie=UTF8&s=books&qid=1284329075&sr=8-2 2. The Object of My Affection is in My Reflection: Coping with Narcissists by Rokelle Lerner. This book is newer but is extremely clear and insightful and has helped people since it came out 2 years ago. Amazon: http://www.amazon.com/Object-My-Affection-Reflection-Narcissists/dp/075730768X/ref=sr_1_5?ie=UTF8&s=books&qid=1284329075&sr=8-5
Many many thanks for your insights, support and compassion.
P.S. I did add a bonus for your first answer. Your second response left no doubt that it was well deserved.
I am one of the people who requires all angles to be addressed in order to rest my mind. I mentioned how Facebook creates hurt feelings, i.e. photos of events to which you were not invited, people who are enjoying a visit in your town but do not contact you, etc. I can't be sure if it's meant to hurt people or it is just insensitivity. I would very much like to hear your view on this.