Wireless sensor networks (WSNs) serving in hostile environments are susceptible to multiple collocated failures due to explosives and natural calamities such as avalanches, landslides, etc., which can partition the network into disjoint segments. In such a scenario, federating the segments becomes essential for reestablishing communication among segments and resuming the WSN operation. Placing relays has been a popular strategy for forming a connected inter-segment topology. In this paper, we tackle the federation problem while considering constrained relay availability. We exploit the use of a limited number of mobile relays to provide intermittent inter-segment connectivity. We propose FLOWER, a novel algorithm for Federation with LOW inter-segment latency and Even Relay load. FLOWER groups the segments into clusters and forms a star inter-cluster topology where each cluster is served by a distinct mobile relay. A cluster at the center of the area serves as a hub to bound inter-segment communication delay. In addition, FLOWER opts to prolong the lifetime of the mobile relays by balancing the energy overhead due to travel and wireless communication. We validate its performance through extensive simulation experiments.