You don't do it to "force" anything. You do it because IT'S THE RIGHT THING TO DO.
Why should the OP's spouse be the only one of the 4 of you affected, NOT to know the truth, and be able to deal with it accordingly as they see fit?
They have a right to know. There are emotional, and potentially legal, financial and even medical ramifications to this other woman, based on what her husband is doing with your wife.
