I need a wireless gadget that may not exist. Pic and description attached

Between the laundry in the basement, the kitchen (sink and dishwasher) and a full bath on the first floor, and a full bath on the 2nd floor, sometimes taking a shower can be interrupted with the cold/hot game that gets played when someone elsewhere in the house starts using the water also. Instead of running up and down the stairs to warn others that a shower will commence, we need a signaling device that the water is or soon will be in use. The house is old and big. Upstairs and kitchen can not hear when the water is already being used. A point of use heater is on the list for later. We don't live with our phones attached to us so a group text would be useless. I think this would be cheap to make. This is what I need. It will be wireless The oval on the left is an on off button The oval on the right is a light. I think it can be the size of an outlet cover. Like a wireless doorbell there will be a receiver but instead of ringing it will pass the signal to the 3 other locations. When someone is preparing for a shower or loading the dishwasher, they can see if the hot water is available. If it is, they press the switch and the light lights up on all the panels. If they see one of the lights is already on, they can wait. So what do I need to make this?