I think your 14yo could be responsible for a younger sib while you eat there at the hotel if the 14yo is mature and normally handles the younger sib well. I would not allow them to go to the pool, just stay in the room. Perhaps order room service for them. I wouldn't go off from the hotell though. Too many things can happen and a 14yo in a strange place may not know what to do.
We left our autistic son in the room with his 13yo sister while we sat on the beach at watched the fireworks for an hour, no problem. We had our cell phones on us the whole time.