Hi Frances, I am José M spanish attorney
If the will of her does not say a word about giving you the right of the use of the propiety ( It is called usufructo ) in Spain, your legal situation is that you have the half of the propiety and the right to keep on the use as you has been using for years. So, if they decide to make cash of their rights they have two ways
1 To deal with you and try to get an agrement
2 If not possible to demand the auction of the propiety in court
The first chance is the best for you, because you can pay less that the market price and solve your problem
The second one is hardest for they, they will have to invest money on it, and the time could be for years in several courts...
In all parts you win after all as it coul not be different and it has to be
So, just wait for an answer of they, when they move, try to get an agrement to buy their part in the price you mean, and if they do not want, let they go to go in court to spend money and time...
I will be here if needed