Sounds as if you may have a failed head gasket. Your brother is no more of a mechanic than you, the temperature sensor is usually in block or head and will trigger the fan sensor whetehr there is fluid or not.

Take it to a garage (after filling up with water) and get them to check it properly.

Coolant loss can be from a lot of different places

and with no other symptoms like fluid under the van or exess white exhaust smoke and with you a paying customer you don't want me doing expensive tests if it were in with me now I do a pressure test on the cooling system and the header TANK CAP.

often overlooked it is designed and meant to vent coolant but only under set conditions.

a faulty coolant cap can have a number of symtoms from overheating coolant to coolant loss.