This is a sore subject and an emotional one, so although I am with easybee here, perhaps the issue should have been handled more tactfully.
Of course, I can talk, as I live in a country where you are neither allowed to have relatives' ashes in an urn in your home, or scatter them on land at all. You may have them buried, urn and all, in a cemetary, by buying a plot, or applying for permission to inter them in a plot you already own, or place them in the cemetary columbarium, so we don't ever have these discussions.
If you want to scatter them at sea, in Danish terratorial waters, you need proper permission from the Royal Navy to do so.
There are two points here whether OP is justified in scattering the ashes without the knowledge of the "difficult" sister and whether she should go and do so, knowing full well
that the place she has chosen is one where scattering ashes is actually prohibited.
The last question is the easier to answer:
No, she should not break the law, however stupid she feels it is. The point is not whether anyone sees her doing so, or otherwise finds out what she has done, or not. It is illegal, so no, do not do it.
How about, OP, sending the urn to the sister who makes impracticable suggestions and giving her your, and your other siblings' permission to do with it what she wants.
Or alternatively, dispose legally of the ashes and either tell your sister what you have done, or if that will cause riot and rumpus, not mention the matter at all.
If she should ever ask about mother's ashes, you will either have to lie and say you have no idea where they are, or no recollection of what you actually have done with them, or own up and face her hurt or anger.