Why would your kitchen sink not get hot water unless you turn on the water in one of the bathrooms?

It sounds like the water heater is so far from the kitchen sink that a recirculating line may be required to keep the line feeding the sink hot, otherwise, it will cool down causing you to have to run all that cool water out before the hot gets there. The reason you get it faster when you run the bathroom is because you are drawing more GPM through the line with two fixtures on.