Wireless Sensor Networks (WSNs) have been researched to detect various events on sensor fields. A lot of sensor nodes in a specific region called an event region around an event detect it and send reporting data to a sink. To reliably detect an event, reporting data should be reliably delivered to a sink in lossy networks such as WSNs with frequent path breakages. Multipath routing can be one of attractive approaches to cope with the lossy networks. However, if many sensor nodes in an event region construct their own multipaths to a sink and sends their own reporting data through the multipaths to the sink, the data cannot be reliably and energy-efficiently delivered to the sink due to concentration of a large number of data. Therefore, we propose an event-to-sink Multipath Routing protocol with Multiple Sources called MRMS that supports efficient and reliable data delivery from multiple sources in an event region to a sink. MRMS decides the number of multipaths for an event region. The number of multipaths is calculated with the number of sensor nodes in the event region, the data size, and the buffer size. Based on the number of multipath, MRMS divides the event region into sectors and constructs a multipath in each sector. Sensor nodes in each sector uses the multipath in their own sector as the primary path and the multipath in their neighbor sectors as the alternative path. MRMS also provides a multipath reconstruction method against the failure of a multipath. Simulation results conducted in various environments show that the proposed protocol achieves better performance than the well-known previous protocol.