In virginia, the executor of an estate has access to a large amount of money to distribute to the siblings. The executor has decided one sibling can't control his money so they are distributing money to him $2000 a month instead of distributing the same amount as the others received. Is this legal?

Depends on the terms of the will and the power invested in the executor. It certainly can be legal under the right conditions.

I know several people that have written their wills a specific way for that very purpose. Its meant to help them, not punish them.

As others noted it can be legal, depending on circumstances. But if the decedent left no instructions to to do this, and if the heir is of legal age is not deemed incompetent, the executor has a duty to distribute the assets of the estate in a timely manner consistent with prudent management of the estate. One issue is that if a trust has not been created to hold the money until its distribute to the heir in question then the executor has to keep the estate open and not finish probate until the assets are finally all doled out. If distributing $2K/month means the estate will have to stay open for more than 6-12 months I think it's a problem.