Ownership of property and Inheritance Tax liability

Hello,

My father passed away about ten years ago and left a Spanish will in the form which I think is the one commonly used in Spain, and which names my mum as the 'usufructuaria vitalica' of the property and my sister and myself as joint heirs.

Even though the house was bought by my parents during their marriage, only by father's name figures in the title deeds to the property (ie the property is only in his name).

We have not yet done probate - haven't yet accepted the inheritance, and not paid any inheritance taxes. I believe that we are covered by the statute of limitation on inheritance tax in spain and since it is more than 4yrs and 6mnths since my father died we are no longer liable to pay inheritance tax.

The questions that I have are the following:

(1) Does the fact that my parents were married when they bought the property entitle my mum to 50% ownership of the property, even though her name does not figure in the title deeds?

(2) If my mum is 50% owner of the property (which I doubt since the property is not in her name) does this mean that my sister and myself would have to pay inheritance tax when my mum passes away and we inherit her 50% share of the property?

(3) Basically what does my sister and myself need to do?

Many thanks.