First, big hugs M.. I know this can't be easy.
Sometimes, with some people, you just can't win. It sounds like your mom has a lot of issues which seem to prevent her from being in a good relationship with her grandkids and their moms (you and your sister). That is very sad.
What I am wondering is if she has some personality issues and is feeling that she isn't getting enough of your attention during visits. Your children have replaced her as your focus, and rightly so. Yet, some people really do not deal with this in a healthy, rational way. When my son was 2, I had an older friend who is very self-absorbed; she would visit and make a lot of passive-aggressive comments that I wasn't putting my son in his place (second to her). This really gave me insight into her limitations and I decided that when I would reach out to her, it would just be one-on-one times. I couldn't change her, that was what she needed from me. Eventually, the friendship fizzled out.
In any case, regarding your mom-- I think this is a loss both you and your sister need to accept and grieve. Your mom isn't being the Grandmother you had both hoped she would be. That IS a loss. You have tried the reasonable approaches (counseling) and it sounds like she's not willing to see past her own perspective of being the wronged one, the martyr.
This is something really important to consider: if your mom does do guilt trips, how much exposure do you really want to give your kids to this? I say this with a lot of my own experience in mind. You don't want to feel bad that your mom won't visit, however, you don't want to feel obligated to make your kids visit someone who isn't emotionally healthy.
What you CAN do: first, accept that mom isn't going to change or live up to your expectations. Next, if you want to do something with her, ask her "give me some dates when you are available" and put the ball back in her court.( It may be nice to do an outing where it's just the two of you, I don't know. That would be one way to approach it.)
And then, DO focus on all of the loving, caring people you do have in your lives. For what it's worth, while some of my family is very aloof, I have one sister who I am close with. Even though we are four hours away, we give each other a lot of moral support. We encourage good long-distance relationships between our kids. We've also created a lot of 'chosen' family for ourselves and Kiddo, so he has quite a few honorary Aunties and their kids who love him. So, even though we are basically alone, family-wise, here in Portland, he does feel very loved.
And when they get older, it's okay to just explain it simply. I've had to explain my mother's not being in our lives (long story, safest option) and just focus on answering the question simply and mostly listening to him, his feelings, and lots of empathetic listening. It really is disappointing when our parents fail us in some ways, the goal for me is to just let Kiddo know that he is still very, very loved and worthy of that love. :)