Assuming the tank is not full and this only happens during the sink draining and then drains down in the shower the flow rate is too high for the hose you are using.
I would try and make a stopper for the sink that slows the drainage rate. The shower is the lowest point and will show any backup first.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles