If you put a stopper in the sink drain and the boat is sailing with the discharge under water it may be possible to get siphoning into the boat. I'm not sure if it could happen with the water rushing by. Some boats not under way have had water coming in through the bilge pump circuit when the discharge was under water. A surveyor may not like your idea, making it difficult to get insurance.

Would it be possible for your sink water to drain into the bilge? Vane type bilge pumps are open pipes. Are you fitting a check valve?