Multicast transfer can efficiently save the bandwidth consumption and reduce the load on the source node than a series of independent unicast transfers. Nowadays, many applications employ the content replica strategy to improve the robustness and efficiency; hence each file and its replicas are usually distributed among multiple sources. In such scenarios, the traditional deterministic multicast develops into the Uncertain multicast, which has more flexibility in the source selection. In this paper, we focus on building and maintaining a minimal cost forest (MCF) for any uncertain multicast, whose group members (source nodes and destination nodes) may join or leave after constructing a MCF. We formulate this dynamic minimal cost forest (DMCF) problem as a mixed integer programming model. We then design three dedicated methods to approximate the optimal solution. Among them, our a-MCF aims to efficiently construct an MCF for any given uncertain multicast, without dynamic behaviors of multicast group members. The d-MCF method motivates to slightly update the existing MCF via local modifications once appearing a dynamic behavior. It can achieve the balance between the minimal cost and the minimal modifications to the existing forest. The last r-MCF is a supplement method to the d-MCF method, since many rounds of local modifications maymake the resultant forest far away from the optimal forest. Accordingly, our r-MCF method monitors the accumulated degradation and triggers the rearrangement process to reconstruct an new MCF when necessary. The comprehensive evaluation results demonstrate that our methods can well tackle the proposed DMCF problem.