Will my father have to honour my mum's trust will?

My father is selling the house he lived in with my mother. Before she died, they both had trust wills made. He is now selling the property as he has moved in with a new partner. Can he do this without the permission of my sister and I who are trustees in my mother's will? And does the conveyancing solicitor need to be informed of my mother's will? My father is being very vague about the situation and I'm concerned that my mum's wishes may not be being considered. Thanks.