My sister is in possession of my fathers ashes and I do not trust her to wait for me to scatter the ashes, she lives on the other side of the country. We are all equal heirs to his estate and she is in possession of my father. Is there anything legally that I can do to prevent her from scattering them before I get there?

Thanks in advance

You could try to obtain a restraining order.

First she is in possession of a urn of ashes, not really your father, his soul, the substance he was, is no longer in those ashes.

And your right to equal share of an estate has no bearing on what she does with the ashes.

And I would say if it is that important to be there, I wouild go, if not what you are staying for is more important to you.
( not trying to sound mean, but just plain direct talk If we were in person I would talk for about a hour to get to that point, but online I don't have the hour)