When willing a person anything, then it becomes part of their estate. What they choose to do with it out of your hands. You can't will someone $5000 and say that the person cannot give it to anyone else. If during probate, a heir dies, then the items are transferred to that persons estate and the estate decides what happens to it.
Basically, you can't say I want to give brother Joe $1000, but if he dies I don't want it to go to his kids.