In O'Dea v. Mitchell, 350 Mass. 163 (1966), the SJC held that a child's right as next of kin vests "only when there is no surviving spouse or no contrary provision by the decedent concerning the disposition of his remains."
As a result of these and other cases, it appears that there is a consistent practice in the Massachusetts cremation industry to require the consent of the surviving spouse, or, if there is none, of all adult children to a cremation.
Obviously, getting the written consent of siblings who may be far apart, geographically or philosophically, can be difficult. The following is from the Funeral Consumers Alliance of Eastern Massachusetts:
That organization recommends that those wishing to be cremated after death obtain a form called "Declaration of Intent Regarding Cremation" from a crematory, so that they can spell out their wishes and appoint someone to authorize the cremation."Order for Cremation or IntermentThis form, required by the crematory or cemetery, is provided and filled out by the cemetery or crematory staff, and signed by the next of kin requesting cremation.Order of next of kin--spouse, or, if none, then children (all adult children must sign, and if sending in authorization by fax it must be notarized and the original sent by mail, if from another country consulate should verify the identity)"
You may also want to discuss your wishes with your estate planning or elder law attorney, so that they can draft the appropriate documents. More information on this and other topics related to death, is available at Mass. Law About Burial, Cremation and Funerals.