A method and device for a source relay User Equipment (UE) are disclosed. In one embodiment, the relay UE establishes a first PC5 Radio Resource Control (RRC) connection with a source remote UE and establishes a second PC5 RRC connection with a target remote UE. The relay UE further receives a PC5 Quality of Service (QoS) flow-to-Sidelink Radio Bearer (SLRB) mapping from the source remote UE, wherein the PC5 QoS flow-to-SLRB mapping includes a configuration index of an End-to-End (E2E) SLRB and at least one sidelink QoS flow identity (ID). The relay UE also transmits a configuration of a PC5 Relay Radio Link Control (RLC) channel to the target remote UE, wherein the E2E SLRB is associated with the PC5 Relay RLC channel. Furthermore, the relay UE receives first information, indicating release of the PC5 QoS flow-to-SLRB mapping, from the source remote UE.