My mother died when I was 3, my father remarried but died 5 months ago his second wife has now died and my stepfamily want to mix dads ashes with hers. I would like some ashes to put with my mother,but the family are not agreeing to this. Is there anything I can do legally to have some of my fathers ashes to put with my mum, and how can I do this.

Saying 'mum' suggests that you are in the UK?
I don't know the law there, but in general, there just are not laws that divide ashes. Where I live, the ashes go to the person who pays for the cremation.
You could try the idea of paying for a 1/2 cup of ashes (there really are quite a bit, at least 2 quarts/litres, because it includes a box), but don't count on that working, if being nice didn't help.

Consult a solicitor. You will have to go to court to force the issue.