It does not have a sister yet! Ermua Municipal Library serves a community with speakers of Basque, Spanish, English, French and Italian. They are very active in social networks. They want a sister library in any of the countries and regions, but especially in Scotland, Ireland or Belgium.
You can find all the information about them and links to their multiple websites in the Libraries Section. Also, by clicking in its name in this post you can see directly its application form.