Does a Will need to be Probated before the beneficiary executes a Will for the same Property?

My parents have a plot of land in Salt Lake leased to them for 999 years. The land is jointly in their names. They had made a mirror will (not Registered) i.e. on demise of either of them, the other will be the sole beneficiary. Presently my father is deceased. No mutation has been done yet and the property still stands in his and my mom's name. Neither has the will where my father bequeathed the entire property to my mom has been probated. We are three siblings. My mother wants to will the entire property to one of us, and the other two don't have any objection to this. My question is: Can my mother make such a will in view of the fact that technically she is only a joint owner of the property as my dad's name still stands as a joint owner and the will that he made in favour of my mom has not been probated? Or in other words, if my mom makes her will and connects it with the previous will that dad had made and mentions that though she has not probated the previous will, will it be legally viable? Or, please suggest the best way she can ensure that after her the property should go to go to the one she wants to give to. Thank you.