Using a non vented Kerosene heater in the house can be safe but Kerosene heaters that are not vented should not be used long term as a heat source.
When you need to use a Kerosene Heater inside that is not vented you should open a window a crack to allow some fresh air into the house.
The Kerosene Heater puts out carbon monoxide which can be deadly if you do not get fresh air.
You can buy vented Kerosene Heaters that can be used as a sole heat source safely in your home but only use the Kerosene Heaters as an emergency heat source if they're not vented.