Gentle ways forward when relatives disagree about scattering ashes at sea: listening, keeping a portion, separate memorials, and who decides in California.
Grief does not make everyone want the same thing. One sibling wants the ashes in the Pacific; another wants a grave to visit; a parent cannot bear to let them go yet. Disagreement after a death is common, and it does not mean anyone loved the person less.
Before anyone argues for a plan, ask each person what matters most to them. Often the real need sits under the stated position. Someone who objects to scattering may simply need a place to go on the anniversary. Someone who insists on the sea may be holding a promise. Hearing the need opens choices the argument hides.
A grief counselor, a mediator, a faith leader, or an officiant the family trusts can help a conversation stay kind. Someone outside the family can say things a sibling cannot. See when to bring an officiant.
California law sets an order. A person's own written directions come first under Health & Safety Code § 7100.1. After that, § 7100 gives the right to control disposition to a health care agent given that right, then the surviving spouse or registered domestic partner (Family Code § 297.5), then the adult children by majority, then parents, then siblings by majority, and further down. For the full order, see who has the right to decide in California. If a dispute goes beyond this, talk with an attorney.
The law can name who decides. It cannot make the family whole. That takes patience, and you are allowed to take your time.
This is general information, not legal advice. Confirm current rules with the cited source or an attorney.