So I just got this email from an in-law:

[Escurumbele]

31. I used to think that not responding to people like that was the correct thing to do, you know, the higher road but

I don't think that anymore. What I would have done in that case is to respond how you were planning to "I am glad you left too.", which in a way is polite, not abusive, but it will eat her up, and the best thing later is to delete all her future emails, so she is going to be left with your last comment, and bullies do not like that.

Anyway, one must do what our conscience, our instinct and feelings at the time tell us to do. You are a good person.