Sibling left an asset that he does not want to share with other siblings

I'm not at all sure those savings bonds are even part of the estate, if your brother's name is the only one on the bonds other than the deceased parent.  Usually, a jointly-held asset like that passes outside the estate, to the surviving owner, and the most common exception, for bank accounts where all the money belonged to the deceased and the survivor's name was added for the convenience of the deceased (to pay the then-elderly person's bills for him, for example), almost certainly can't apply here.

If one or more of the other siblings' names are on the bonds, you'll have to go to court, to force your brother to cooperate.  Otherwise, you probably have no claim on the money anyway.