Increasing popularity of XML and P2P networks has generated much interest in distributed processing of XML data. We propose a novel solution organized around a mediator capable of processing XQueries over multiple heterogeneous data sources. The solution consists in a hierarchical overlay network formed of peers and superpeers indexing the XML path of data sources. Our solution is: (i) Scalable as the network can grow dynamically with size adaptative trie-based indexes in each super-peer. (ii) Reliable as procedures are developed for recovering from node failures or root saturations. (iii) Efficient as query processing is done by an existing optimized mediator that can track query process progresses and response sizes.