When BM was forced back to WA (she was told she needed to come back or loose custody) she chose to stay 1 1/2 hours from us. She was told in the inital GAL report that if she continued to live 1 1/2 hours away she would be fully responsible for the travel. It was an empty threat under our state laws we found out later and it reverted to the receiving parent being responsible for the travel. We would pick up on Fridays and she would arrange to have someone pick up SD on Sundays.
In our case BM very seldom was the person to do the traveling. It was either her new internet boyfriend (who married her and moved to WA during the court review), or BM's parents.
This also was very irritating to my husband who was told by BM that she would not allow me to pick up SD, but yet she never was the one to come and pick her up at the end of the weekend.
Our GAL told her that I should be allowed to do the pick up, but I seldom did it just because it irritated BM so much.
MN has a new law about out of state relocations that put the burden of proof on the custodial parent wanting to move to prove that there is a legitimate reason for the move and then the court makes a decision based on the best interest of the children.
I don't know about what's gone in your case. Has your husband always had custody? If not, what happened that BM lost custody? How old are the kids? How well have they adjusted to the move? How well do they do with the long distance travel they are doing every other weekend?
In our case, after 2 years, SD would tell us how much she didn't like having to do so much traveling. It made her fell like she did not want to come and see us.
Obviously BM is frustrated. What is it she really wants? Can she move closer to you to be closer to the kids? Can you move back some time? Does she need acknowledgement that having her kids taken so far away from her was really unfair? Does she feel like you are "stealing" her kids from her? Can you arrange more phone contact, maybe "visits" fia the internet or talking on IM, more pictures, more letters and pictures sent to her from the kids, adding time during the school breaks?
There are lots of ways you can support the kids' relationship with their mother. If she sees some effort and acknowledgement that she is an important part of her kids' lives, maybe she won't be so angry about the travel.
Seems like you are making a reasonable effort to help with the travel. Makes me think it is just the tip of the iceberg of what she is really mad about.