With respect to sideslip on the ground, when taking off in a crosswind there will be sideslip (although, obviously, you cannot apply it). A crosswind from the same side as the failed engine will increase the minimum control speed and vice versa. Remember that Vmcg is a specification definition speed that has a maximum crosswind component stipulated (from memory, it used to be 7 kts for BCAR but I think is nil for CS25). The actual minimum speed at which you can control the aircraft may be greater or less than Vmcg according to factors such as crosswind and derated thrust.