Can I put what may be a different spin on this situation? How much do you cherish your marriage? I would bet its a lot. If this continues, if you allow this continue, you are great risk of destroying your marriage.
What good is family if they only use you? if they're going to take the side of the user and manipulator? You have to understand that I come from an exceptionally dysfunctional family (both maternal and paternal, going back generations). I finally walked away from all of them but my own sisters. Even then, there is a wall that will never come done because I simply cannot allow the dysfunction to continue in my own family. MY FAMILY, my husband and my children, come first. No mother, brother, sister, grandmother, aunt, uncle or cousin is going to be allowed to do ANYTHING to hurt them. Even if that means a painful decision is made.
Give them 30 days to get out. Now. Kick Mom out of the LR. It belongs to YOU and YOUR HUSBAND.
The only thing I might do is buy the ring from her NOW before she pawns it. At least then you'll know it's safe. Otherwise, DO NOT GIVE HER or HIM ANYMORE MONEY!
{{{{{ hugs }}}}}}