Steven Covey has a beautiful visual that may help you with this. Helps me all the time.
Picture a larger circle and a smaller circle within it. The larger is your "circle of concern" (all those things that concern you....how the girls are feeling would be there). The smaller one inside of that is your "circle of influence" (what YOU can actually do/have influence over would be there....ie: how you show love to the girls at
your house).
We get into a real mess when we are out in our circle of concern, spinning our wheels, obsessing and worrying about things over which we have no real control at all. It's a recipe for pain.
In spending our time "out there" we actually diminish our smaller inner circle of influence, as we are wasting our energy, and not finding creative solutions where they can actually have some effect.
If we turn our attention, instead, to that which we CAN influence and spend our thoughts and energy there, we actually find creative solutions to our problem, increase our influence further AND we are a lot happier.
Now I ask you, what could be more important to those girls than spending time in a home that is HAPPY?
Make sure your intention is pure. Make sure you are not just a second lieutenant to your husband's anger at his ex wife. If those anger issues are cleared and your concern is truly for those children, then focus on what will help them the most. BE happy. Stop concerning yourself with the myriad of problems at the mom's house. You cannot influence that.
As the children get older, the natural narcissistic tendencies of youth will be replaced with making a choice of how to do life. They will choose the example presented to them that shows happiness, positivity and mental health.
That's what you can do for them.
My children's father is self centered, narcissistic aand manipulative. His wife is rude to them, and they both drink too much. What can I do about that? NOTHING. I can only have influence when the children are with me...they can see me move on and make healthy choices in life. That's all. If I don't like their behavoir at my home, I can set boundaries and consequences. If they are learning it from dad's house....oh well! I can still only do what I can at my house to be safe and happy.
I will admit to bristling at your comment about going after a 'parental alientation" judgment. PAS" is subjective, sticky, politically generated and hard to prove AND disprove. There are many, many cases where it has been used horrifically by the "father right's initiative" to give custody to abusers... Obviously you are not of that ilk, but it's a real nightmare, that PAS. Like playing with poisonous snakes. I hope your husband has moved on from that.
Stick with the positives, not the negatives. You love these children. You offer them a good example of how to behave and demonstrate love to fellow humans. Ignore the mother's "inadequate" mothering and N ness and stay within your circle of influence. What goes on in your house. What kindness looks like. What healthy behavoir and expectations look like. What forgiveness looks like. What love really does for a person. Those will be mighty lucky kids, and I must say, they are already, for all your apparent love and caring!
Focus on what you WANT for these girls, not what you DON"T WANT. Then you will be increasing your own happiness, and demonstrating a great way to do life for these girls.