Final answer:
The nurse should provide emotional support and empathy as the first intervention when family members ask questions about their critically ill mother's prognosis.
Step-by-step explanation:
The first intervention the nurse should implement when five family members arrive at the hospital and begin asking questions about their critically ill mother's prognosis is to provide emotional support and empathy. The nurse should acknowledge their concerns and offer reassurance while explaining that the medical team is working to provide the best care. The nurse can also encourage open communication and suggest designating a family spokesperson who can coordinate updates from the medical staff to ensure everyone stays informed.