I know of no way to force grandma to give son her money before she wants to. Generally, life insurance proceeds are not part of the marital estate because they are not acquired during the marriage -- the proceeds appear only after death, and marriage does not continue beyond the lives of the parties involved. It may be that grandma believes she made a mistake in giving sister the money when she was 18, and doesnt want to make the same mistake twice. 15 year olds often believe adults are trying to control them -- and many times they are. If there is a real need for the money, certainly, grandma should be asked for some of it now. Or, if it is needed for college or similar reasons, grandma would likely provide at least some of it before son is 21. You may be able to sue to claim that grandma holds the money in a constructive trust, but I very much doubt you would get much relief, especially if the money is still intact and hasn't been absconded with by grandma.