No judgment. Personally I am not into family members and to me it seems that it would raise all sort of issues not only in your marriage but at family gatherings.
FIRST check with your spouse to see how she feels about having her mom join you in that way. If your wife isn't okay with it I certainly wouldn't cross that line. But if wifey is comfortable with it hell have a wonderful 3some and enjoy both versions of your wife. The young present wife and most likely what your wife will look like in another 20 years in your mother in law.
Good luck and happy humping.