Every house has a valve that stops the water. In an emergency, the person who knows where it is may be the only one who can use it. Keeping that knowledge current, and keeping the valve working, is an ownership job, not a one-time chore.
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Find it and write it down
Walk the house with whoever usually helps her and find the main shutoff. It may be in a basement, a utility closet, near the water meter outside, or along an exterior wall. Write the location on paper, add a photo, and keep a copy inside a kitchen cabinet and another in the family's shared folder. Do not rely on memory or on the person who built the house, since they may no longer be around to ask.
Test it each season
Once a season, turn the valve slowly to closed and back to fully open. Go slowly and do not force it. A valve that will not move, drips when closed, or grinds badly is not one to rely on. Call a licensed plumber rather than working it harder. Log each test on the same sheet as the location, with a note of anything that felt stiff or changed.
Name the owner
Decide who is responsible for the test. It might be the family member who visits most often, a hired helper who has been asked to do it as part of their written duties, or the property owner if you rent. Put that name on the house sheet next to the location. When each new helper starts, ask whether they know where the shutoff is and what they would do if water came through the ceiling.
What it takes to keep it working
Keep the valve area clear of boxes, cleaning supplies, and anything that would block a hand or a wrench. Check it for damp spots, rust, or a slow drip at the stem. If the valve needs replacing, that is a plumber's job, and the house sheet should record who was called and what was done, so the next owner of this task starts from the facts.