The story thus far:
Mary has died leaving Al in a house which they co-owned 50:50. Al has had to give up work to look after Mary and with Mary's death their income has dried up. Al cannot afford to live in such a big house, she cannot afford a loan with which to buy out her sisters, Mary's estate has no money with which to buy Al out.
She looks again at Mary's will.
"this clause shall have effect so as to give [Al] the option of buying my interest in the Property at the Price [90% of its value]
"WITHIN six weeks of my death my Trustees shall give notice to [Al] of the of the option granted by this clause and take the steps necessary to have the Price determined and within four weeks of that determination being communicated to them shall give notice to [Al]
"WHEN the Beneficiary has notice both of the option and of the Price she may within ten weeks give notice to my Trustees of her wish to exercise the option?
In Dec 2004 the house is valued, in Jan 2005 it is offered to Al, it sells in early March with the garden is full of daffodils, and on completion day in May/June Al exercises her right to buy her sisters out.
Could that work?