In a high tower

A queen and her son and daughter were locked in the top room of a very high tower. There was a pulley with a rope around it right next to their window, and  baskets, each fastened to both ends of the rope. Both baskets weighed the same. When one basket was at the top of the tower, another was on the ground. The prisoners managed to escape from the tower using the pulley and baskets and a weight that they found in the room. But how?

If one basket weighs 10 kg more than the other one, the prisoners would move downstairs so quickly, that they could get injured. When one basket goes up, the other certainly goes down.

The queen weighed 90 kg, daughter 50kg, son 40kg, and the weight - 30 kg.

What is minimal number of steps they have to take so that all canescape safely?