You cannot know the answers, I believe.And why pick your mother to steal from!!!
Your task, my task, is to just face what has happened. To stop for a second. And look reality in the face.
She stole 18k from you. Money left to you by your own mother. Money that was meant to protect your family, or secure your retirement, or for simple pleasures.
Why she did it is not your task to know. I said above, that her wanting to "explain" troubles me more almost than what she did.
I do not think she knows what she did or why. Anything she says now is not anything she feels or knows. How could it be? Perhaps she feels shame. Perhaps she feels regret. Perhaps she feels discomfort and sadness that she has lost the confidence she had taken for granted. Maybe even anger. She may feel it was a mistake, and that she needs to be forgiven because she did not mean to hurt you, in the main or primarily.
She may not be old enough yet, without a sufficiently mature brain to understand how she has hurt you, damaged the family. Maybe she picked you to steal from because you were easy prey. Maybe she felt it was low risk. Maybe she knew you would not report the theft to the police.
None of these things, you want to hear will make any of this better. There is not one thing you can come up with that will take the pain from this away. Only, to face it.
She did it. She will either one day understand it or not. She may one day move beyond it. But your understanding why will have nothing to do with it. Only what she labors to do for herself.
I am trying here to think of my own situation.
All I feel is grief.