The question of whether IHT will be due depends on several things. Firstly, which region of Spain your house is in. Different regions give different allowances against IHT.
Secondly, is the house your habitual residence or a holiday home? The allowances can only be claimed on a habitual residence.
Thirdly, how long have you owned the house? In Andalucia, for example, you need to have owned the house for at least 3 years as the habitual residence in order to qualify for the regional allowance against IHT.
It will be in your interest to get this sorted out quickly as if Hacienda is not notified of the death, and any tax due paid within six months of the date of death, there would be additional penalties for late payment. You would need to consult an abogado who will calculate whether there is any tax due, complete the necessary paperwork for Hacienda and once any tax is paid, draw up an Escritura de Herencia (deed of inheritance) and have it notarised.