I think DH is going to talk to his father today. No doubt the double life he led did create a lot of tension. It is a little different than your average divorce because so much deceit occurred not only to MIL, but to my DH and his sister as well. At one time FIL even used an alias and was listed that way in the phone directory.
I think if his phone message had not sounded so aggressive, we may all have been more receptive, but to hear that tone was a huge turnoff. We have tried for so long to keep DH's family together in some semblance, but it has not been easy. FIL has left this woman several times and at one point was very fearful of her as she had pulled a gun on him. We - meaning DH, DS and me - just have a hard time accepting her and welcoming her in after all that has gone down. Plus we haven't seen her in years. The last time FIL left her was only about a year and a half ago.
Many of you may be right that if you invite one, you must invite the other. But it does seem the precedent was set by her not coming around for years and he does come around without her. But now that there is a wedding he expects us to invite her. Maybe we are rude and have no etiquette. I just think he is picking the wrong day to expect us to have some reconciliation with her. Why does it have to be DS's wedding day?