At two in the morning the worry is usually small and specific. She ran a bath, or washed her hands at the kitchen sink, and the water came out hotter than you would have let it. Nobody was hurt. But now you are asking whether the house can keep her safe at the tap, and whether that is a job for your own hands or for a plumber.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
What makes the water hold
Most of the answer sits in two places. The first is the water heater, which has a temperature setting, often a dial on a gas control or a thermostat behind a small access panel on an electric unit. The second is the mixing or anti-scald valve at the bath or shower, which blends hot and cold before the water reaches her. A house can have one, the other, or neither, and you cannot tell which from the front of the faucet.
Start by reading the heater's own label or manual. It names the setting range and what each knob position does. Then run the hot tap at the kitchen sink for a minute and check the temperature with a basic thermometer. Write down the number and the time. That single reading lets you and the care team talk about the same thing instead of guessing.
What you can adjust yourself
Turning a heater dial down to a lower setting the manual allows is a task many families do without help. It is a change you can undo in a minute, and it does not open any lines or walls. Leave a note on the heater saying what setting you chose, so the next person who touches it does not turn it back. If the unit is gas, read the instructions printed on it before you touch the control, and do not relight a pilot unless the label tells you how.
When to hire the plumber
Call a licensed plumber when the reading stays high after you have lowered the setting, when the heater has no setting you can reach safely, when the water still runs too hot at the tub after the heater is turned down, or when you need an anti-scald valve installed at the shower. That valve sits inside the wall and ties into supply lines, so it belongs to someone who can pressure test it and check the work against your local code.
Call the gas utility first, not a plumber, if you smell gas near the heater or see the tank leaking at its base. Those are not settings questions.
What to ask before anyone opens a wall
Ask the plumber three things. Will the valve be tested at the tub and shower after installation, and who signs that test? Will the work include a written temperature reading before and after? And if someone other than the manufacturer's installer does the work, what does that do to the heater warranty? A good answer is specific and written down.
The skin question belongs to her care team
If you have noticed she no longer pulls her hand away from warm water, or that she has stopped mentioning heat the way she once did, take that to her doctor and her care team. How she feels heat is a medical question, and this page cannot answer it for her. It is still worth raising while you check the tap, because both concerns come up in the same bathroom.
For neighbors and services that can come to the house, see local help for keeping her home.