There is a company handling all cemeteries in Barcelona. Its name is Cementiris de Barcelona.
From their website:
Cementiris de Barcelona, S.A. is part of the B:SM Group. It manages the new cemeteries in the city of Barcelona, Montjuïc, Poblenou, Sant Andreu, Les Corts, Collserola, Horta, Sants, Sarrià and Sant Gervasi, as well as the two crematoriums situated in Montjuïc and Collserola. Its main objectives are to focus the running of the cemeteries and all their activities on the users, to improve personalized attention, to provide quality funeral services that involve the transfer the deceased persons to our cemeteries, and to disseminate the culture and the history of the cemeteries.
If you browse through it you can find a section Services → Location deceased in which a form is displayed so that you can look for a deceased.
I don't know how well this can work, but it definitely works like the place to start looking from.
If you know where in the city was your relative living, you can more or less guess where she was buried. However, as you see from the quoted text, there are not that many cemeteries in the city, so you can do search for your relative's name in each one of them.
I did a basic research on a very common Spanish name, Carlos Garcia, in the cemetery of Montjuïc and got a nice page with 28 results
The page results are in Catalan, but it is not difficult to understand:
Premi al damunt del difunt sel.leccionat, d'entre les 28 semblances trobades:
Cognoms, Nom Núm Pis Tipus Agrupació
Which translates to:
Select one of the decesead from the 28 matches that were found:
Surnames, Name Number Floor Type Group
When you click the name, you get to a page with a very complete map on the position of the grave within the cemetery.