help with protective property trust will

I wonder if anyone can help me, my parents made a protective property trust will a few years ago to help protect there assests from nursing home fees, since then my father has died and my mother has decided she want's to go into sheltered accomodation. The trust has been set up so they are tennants in common and me and my sister are trustees I think either of them are trustees also. The trust states that me and my sister a end beneficiaries 50/50 and my mum has a life time interest. The trust as it stands has my fathers half held in it. Right, sorry to be so long winded, the problem that's arisen is that my sister is now abusing her trustee postion by threatening to put her son into my mums house when she goes into the sheltered housing and only charge him a small rent, she is also trying to persuade my mother to sell the house to her son at a discount rate in six months time, my mother is quite vulnerable and goes and tends to go along with her. I think the property should be rented at a proper rent to pay for my mothers rent and help with her living cost. What are my options as co-trustee to protect my mothers interest.