I don't know about "rights", but people absolutely can exert their will from beyond the grave: think of wills, estates, etc. That's a good thing - It's not the state's business to redistribute your family's assets as it sees fit.

If a person didn't make a choice, their family should have the final say... They certainly have more of a right to the deceased's remains than the state.

To me, the central issue is about choice: Who gets to make choices about my body? Choice matters. The whole point of life is to make choices.