British pension paid abroad.

No, they will not stop your UK pension just because you take out citizenship. Are you taking out citizenship in Spain? In which case you need to have been a legal resident for 10 years or more, take a test in Spanish regarding current affairs and the history of Spain. This 10 years is for those who are foreigners and have lived here (not in the EU) for those 10 years but can be reduced if you are married to a Spanish citizen or come from a country with Spanish ties such as those in S America. 1 year on marriage and 2 years with Spanish ties.

You then need to attend an interview, in Spanish, showing how you have integrated into Spanish society and they will be looking for things like participation in Spanish social activities. 

You can then apply for a Spanish passport and must pledge allegiance to the King and constitution of Spain and, in most cases, relinquish your previous passport as Spain does not recognise dual citizenship. But it will not affect your entitlement to a pension from any country you have worked in.

I have actually gone through the citizenship process but baulked at the end with the allegiance and renouncing of previous citizenship at the very end of the process. One of the questions I tripped up on was knowing the names of the King of Spain's two sisters, I knew Cristina, his younger sister, but could not recall his elder sister, the Duchess of Lugo Infanta Elena. Just to let you know the type of questions you could face. 

It's true that Spain does not recognise dual citizenship but it's a paper rule which does NOT mean relinguishing your UK passport, which belongs to the UK government anyway, or actually renouncing UK citizenship.

My sister has a british and an eu passport you can keep both.

Is your sister's EU passport a Spanish one?

Different countries m in the EU have their own rules in this matter.

J