Ok speaking from experience here you have to know that when people stray from a marriage its because something is lacking in the marriage. So it is everyones fault. You need to have lots and lots of communication with your husband. You need to know why he felt it necessary to write back. I...
I wouldn't even waste your time going to sit on the hillside. If you do and she see you sitting there she will gloat and know she is making you uncomfortable. I would make other plans wiht your husband to go on a romantic evening/weekend by yourselves. Don't make yourself sick over this work...
I agree "when hell freezes over". You have to keep your family safe from this person and if that means not going to the wedding then more power to you. Your SIL may be putting her own marriage at risk if she allows this knowingly and all.
What ever you choose though best of luck to you and...