Thanks to all.
I tried just having a cabinet in the project, but the result was the same.
Editing the sink outside of the cabinet works, but the sink then becomes a stand-alone object which then can't be properly placed in the cabinet.
katalyst777 Thanks for the info. It is, as I suspected, not a supported feature in Interiors. I can live with what I have got, if it costs over $100 to fix it!