Re: Will it be the appropriate question according to the answer?
Maybe this is an American thing, but I can easily hear myself saying "I"m going to my sister's for the weekend." "I went to my parents' last weekend."
I had never thought about the use of the apostrophe, since you don't hear it when you say it and I'm less likely to write that experssion than to say it.
Perhaps it's a bit like the "greengrocer's on the corner" example we had recently.
HOWEVER - for your writing exercise, I would encourage them to write "his parents' house" as suggested above, or "to visit his parents" and leave out the house but not making it possessive.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.