"The Ken's house" and "the Hemingway's novel" are both ungrammatical - the rule seems valid for singular proper nouns -but other such constructions are possible, for example: We went to the neighbors' house for lunch. We went to the professor's house for tea. We went to the Joneses' house for dinner.
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CliveIn what kind of context did you see this?I don't have any context on hand at the moment. Maybe my memory doesn't serve me right.
Then is thisOK?
We went to the Hemingways' house. (two persons)