That's not true. Remember the tennis player who was visiting friends in Long Island. Same thing happened. He was sleeping out by the pool.
The probability of two related people in the same room dying of heart attacks is very low. Anybody with half a brain would look for an environmental factor.
The world "common" means it occurs frequently. Frequently generally means more than once or twice a year in a population of 300m people. It also means that this type of death is universally known. For instance, motel pool drownings are common, and occur on a monthly basis. A simple Google search unearthed a large number of distinct listings for motel pool drownings, whereas a similar search for motel pool heater deaths unearthed the same incidents over and over.