IncogNeato:
As I See It:She might be a great bridge partner and fascinating conversationalist, but she's not someone I would make a guardian to a minor child.
If one or more of the LW's siblings were not yet born when the LW was 10, and the LW is now perhaps 28, then the mother IS the guardian for at least one minor child. A child who no doubt depends upon both his mother and father for moral and emotional guidance, flawed though it may be. And yet, you encourage this LW to share information with that minor child which is likely to upset this minor as much as it did her.
Even if the minor is 17, it could upset him as much, because it tells him that Mom and Dad may have had him as proof that they had forgiven each other, or as glue to shame the other into not leaving, instead of being created because they wanted him. It would be very similar to telling a kid that he was an "oops" and should never have been born at all. A kid getting ready to finish finish high school and start college or living as a working adult doesn't need his foundation shaken like that.
Nice try, but if you read my posts I have consistently said that I advise discussing the situation with the NEXT CLOSEST IN AGE--precisely to determine whether any child living at home is at risk. When I said I would confront mother, I didn't mean in front of the whole family, but in private.