Don`t worry jasmin. Nobody is going to insist that your sister cancels the holiday. Insurance companies require the customer to made a medical declaration if they have any kind of condition that could affect their cover. The questions that are asked are standard across all companies. Some medical declarations can be done online and some are phoned in. It depends on the insurance company. The main concerns of insurance companies are conditions where the customer could keel over with a heart attack or stroke (high blood pressure, high cholesterol etc). When your sister declares her cancer, she will be asked how long ago she was diagnosed, whether her disease has spread to the lynph nodes or beyond, what kind of treatment she is getting etc. They will give her a price for her insurance based on the answers she gives. She certainly won`t be uninsurable (unless she is in really dire staits medically which is doesn`t sound like she is). She`ll probably just have to pay an additional premium.