That's every parent's nightmare. Kids, even adult kids, are unfortunately an easy prey for the manipulator. You can talk to them, you can try to make them see your point of view, but if they've been "groomed" by your mother, they won't listen. In fact, they might become even more loyal to grandma.
All you can do is let it be, and hope that with time they will see the truth. Because no matter how good a manipulator she is, the truth always comes out eventually.