Not exactly step related but I could use some opinions
My FIL has been dating a woman I'll call Betty for 10 years. We've become really close. Over the years he's run around with other women and it has been super painful for Betty. One woman, I'll call Sue, has been in and out of his life for over 30 years. FIL actually claims her daughter as his own, giving her his last name but isn't biologically his. Sue and FIL dated while she was pregnant 30 years ago. I've only met Sue a handful of times. So, Sue starts coming back in the picture and meets up with FIL more often than Betty can handle. Finally FIL and Betty break up. She and I talk on the phone all the time, almost as much as my own mother and I do.
FIL is planning to marry Sue and he hasn't told Betty. I feel like I'm lying to her all the time because it is a secret I'm keeping for FIL. She is in a lot of pain and truthfully I think knowing would nail the lid on the coffin for her. I've explained to my FIL that what he does with his life is his business and that I will welcome Sue with open arms. However, I have a strong relationship with Betty, and although separate, I feel like I am lying to her by keeping this secret. I've asked FIL to tell her and he said he would. He called and left one message and never called again.
So bottom line, the wedding is in a few weeks.
Here's the question:
Should I tell Betty, after it has taken place since he doesn't seem to have the courtesy to do so, or do I leave it his secret and continue having an elephant in the room every time I talk to her? When she finds out, I"m sure eventually somehow she will, I"m afraid she'll feel I've betrayed her too.
Please give me your opinions. I'd like to have un-buffered opinions from lots of view points and that is hard to do in my own circle.