how much iht do i pay

My father died five years ago.
He was married to a woman who was not my mother and never adopted me.
The joint will left the house to the woman who then leaves it to me and her children.
She has three daughters in the uk.
When she dies the property is to be split - half to me ad the other half between her three daughters.
The property is worth approx 150000 euros. so I would get 75000 euros.
How much tax would I have to pay. I am resident in Spain.
Also can she change the joint will now my father is dead and leave it all to her daughters.