A father died was cremated and his brother will not give or share the ashes with his two children . The children are 14 and 8

Who is the legal guardian of the children ?

Who is over the estate of the deceased person.

I don't know of any US states that have laws deciding how ashes are divided. Where I live, they are generally given to the person who pays for the cremation, unless special requirements were stipulated in the will, not common. This is a matter for the children to work out among themselves, but as minors, the 14 and 8 year olds don't really have any rights.