Hello mike,
As mixed node and model node are of different classes so you can not type cast between those two. As praveen suggested first get the model node from mixed node by using IF_BSP_WD_EXT_PROPERTY_ACCESS~GET_MODEL_NODEof cl_bsp_wd_mixed_node class and assign it to cl_crm_bol_entity . Then execute the get_related_entity/enties according to your requirement. Hope this will solve your problem.
Regards,
Santosh