A life interest can mean many things- a right of residence, an exclusive right of residence, full ownership of the property until their death after which it passes to the others. It matters a lot what kind of life interest it is.
Assuming the life interest is a right of residence (not exclusive), the resident has no obligation to pay rent. This is because all four siblings are entitled to live there, why on earth would any of them become entitled to receive rent because they choose not to exercise that right?