Hi Pete
The egg layers my MIL got never get broody due to selective breeding, but the regular Thai chickens we have certainly due. They lay around 8 to 10 eggs, and then that's them out of the game for a few weeks. They also get quite aggressive

and will really have a go at you when you interfere with them.
I've found you can sometimes break their broody spell simply by removing their nesting box. Then again, if they spot another nesting box with an egg or two in it, they'll usually just move into that one.
Your best bet, if your sister is in no big rush for eggs, is to do as our man from down under says, and give the hen some fake eggs. You can buy fake eggs but they aren't at all necessary because even golf balls work, and if your sister wants to have a bit of a laugh she can use different color balls as well.

I am being serious by the way - the balls can be any color. Tell your sister to still check the nest daily though because other hens will most likely also lay their eggs in the same next.
If you sister determined to stop the hen from being on strike, she can try keeping it in a cage with no nest box for a few days.

Don't try to impress me with your manner of dress cos a monkey himself is a monkey no less - cold fact