When you take a shower and you lose hot water when someone else in the home runs water, the problem may be that your main water pipes are too small.
New pipes might need to be installed in the home.
Or, there could be sediment buildup or corrosion at the opening of the water heater hot water valve.
Sediment or corrosion could cause fluctuations in the water temperature.
A professional plumber should be called to asses the situation, since solutions could include replacement of corroded pipes, checking the water heater, or replacing the shower system with one that is pressure balanced.
The best way to determine the source of the problem and ensure that you have it repaired correctly is to contact an experienced plumber.