Poor mama. You're right, it sucks, there's no other way to describe it.
I'm in the exact same boat. I never thought I could fall in love with that three year old kid so quickly, but I did. This Saturday he goes back to his mom after spending the summer with us, and I can't imagine how my fiance and I will deal with it. Him especially. When Gavin went away for a week to visit his grandparents earlier this summer, my ordinarily happy-go-lucky partner flipped into the worst mood I have ever seen him in. He warned me ahead of time that this weekend will be even worse. I feel so blessed that this great little boy has come into my life, along with his equally wonderful daddy, but it makes it that much more painful that we can't all be together.
I don't know what we can do, besides give them space to be sad and angry, and recognize that the issue has nothing to do with us, and everything to do with the fact that they terribly, achingly miss their children.
Is there any chance you and your partner could relocate to be closer to her? I think that's the only true solution.