Martin Kochanski wrote: > To expand: "su" can mean "his" "her" "their" as well as the > polite "your". In this context, "el marino, el hermano de su > madre" risks being felt as a complete phrase in itself (the > sailor, the brother of his mother), so you need "de usted" to > anchor it firmly to the second person. [...]
But, if the problem was just avoiding the ambiguity, he could simply have dropped the ambiguous pronoun: "el hermano de *la* madre de usted". So, I guess that the double possessive "su" + "de usted" is for emphasis. Alternatively, it mimics a conversational situation: he says his sentence, realizes it is ambiguous, and adds in a hurry an ungrammatical specification. But this would probably have been marked by punctuation: "el hermano de su madre... ˇde usted!... el que despareció." BTW, I wonder, isn't all this sliiiiiiiiiiiightly off topic? :-) _ Marco