Concerned Relative

My sister had made my brother the trustee of her estate. Within 10 days of her passing, and under opioid pain medication, she amended her will as follows: 1/4 to be distributed to our Mother with him as her trustee. Does this mean he can withhold all Mom's portion if he so deems? Where does that money go if Mom passes? How long does this money sit there for her to use? So many questions this has brought up...Thanks for any help you can provide.Helen