First, I would view what happened differently than your husband. It wasn't right that she told the great aunt he was involved when he wasn't. Perhaps if she hadn't said what she said, the great aunt wouldn't have given her the piano for their children. Perhaps she knew your husband wouldn't agree with her having the piano. At least it appears that her heart was in the right place. She wanted it for your step-children and she did provide them with lessons.
Therefore, I'd talk with her about passing the piano down to the next set of children. I'd work on changing my attitude to one of being conciliatory, expecting to improve our relationship. We are more likely to see the good in people when we expect good.
However, know that moving a piano that distance usually causes it to go out of tune and tuning it again is expensive. If the piano is old it might not even be able to be tuned. I moved my great grandmother's piano from a city 3 hours away and the piano tuner said that the strings were too old to allow it to be tuned again and restringing it would cost thousands. He said it wasn't worth tuning. It had been out of tune for years and that might make a difference.
If you're moving the piano 6 hours then just paying for the move is cost prohibitive. We'd planned to move our family's piano ourselves but were advised against doing that. We hired professional movers. Too bad we didn't have a piano tuner look at it first. :(
Anyway, I also suggest that you rent a piano and see if your sons are interested in learning. I rented a piano for my step-son who was very interested in music and he didn't like the piano. I was glad I'd just rented.