If your brother left no will, then how can the ex claim to be the administrator of the estate? If someone dies intestate, then there would need to be a probate hearing to determine if an administrator should be setup and who that would be.
Check out this booklet:
http://www.cfb.ca.gov/formspubs/crd_booklet.pdf
Scroll down to page 11. According to what I read, there is NO requirement that ashes be spread or interned.
§ 7054.6. (a) Cremated remains may be removed in a durable container from the place of cremation or interment and kept in the dwelling owned or occupied by the person having the right to control disposition of the remains under Section 7100,
So already the ex is giving you incorrect information. As next of kin, it would seem to be that your mother had the right to receive the ashes. Barring a will giving instructions on the disposal of those ashes, she can do with them as she pleases.
Has the ex actually started some court proceeding? If no, I would go back to her and cite the section I cited and tell her, since the law clearly gives you the right to keep the ashes, you don't understand what she is talking about. Also ask her who appointed her administrator of the estate? I would then tell her, that you would be happy to discuss the disposition of the ashes with the boys and only them.