When one inherits in Spain, say a property or part of one, they must pay the tax due before they can take possession and sell the asset.
For non-resident inheritors the tax free allowance is I believe around 17,000 euros per person, after that there is a sliding scale of tax liability, which also involves calculations based on the assets already owned (anywhere) by the inheritor. The rules also vary between autonomous regions.
Contrary to the advice I would normally give, I suggest you consult a legal professional, as it is I believe a potential minefield.
PS
My mother has done everything"properly" but won't really discuss it with us.
I don't know how you can be certain of this. I would suggest that you find out now what has been done, who has done it and confirm that is is all in order. Otherwise it will be you paying the bills (and arguing with your sibling) to get it sorted.