If you want to know why she's contacting you, I think it would be perfectly okay to ask her. I think it's normal to be curious when someone contacts you out of the blue like this.
You could PM back to her and say something like, "After all these years and after what you and my father did to my mother, I am quite surprised to hear from you. Why are you contacting me now?"
I see no reason to give her a pass on her prior behavior. You can politely make her aware of your discomfort with her message and at the same time inquire as to WHY she's contacting you now, if you are curious and really want to know.
Block and ignore is good advice, but it's not very satisfying if you are a curious person. You can make your case and still be polite. If, after you've PM'd her, you can still always "block and ignore" her.