She needs professional help
So id bring her to therapist

Replace the carpet.
I Also hid the knives for about ten years.

Make her buy new carpet ?🤷‍♀

She’s a danger to you and your family.

Have her unvestigated for arson?

Talk to her. Ask her why. If she won't open up to you, then she needs a therapist.
Has she had behavoiral problems before ?

Context? Was it intentional or an accident? How did it happen? Was she present when the fire was put out or did she leave something that caused fire unattended?

I burnt a spot in our carpet when I was about 20 too. It was an accident.

Not enough information.
How, why, remorse?

Certainly needs to contribute to a carpet replacement fund.

That’s arson get her punished so she’ll learn

I'm presuming it was accidental... Learn from it

By accident, I hope ? That would the difference between a caution to be more careful vs. getting professional help. 🥺