Twin sister help

We'll call her Dawn. She's in her mid 50's.

Dawn is my wife's twin sister.
She was the second one born.
They have a younger brother.
So the siblings referred to Dawn as the middle child throughout their younger years.
Dawn says my wife was their dad's favorite and the younger brother was mom's favorite and that she has no one.
They went to counseling as a family when they were in HS "for Dawn".
Obviously there was some family issues in their younger years.

Wife and I have been married for almost 30 years now. Very much in love and happily married. First 5 years were very tough though. We both admitted we weren't perfect and have been great over the last 25 years!
Dawn ended up marrying a guy who got her pregnant.
While Dawn was pregnant and unmarried she asked us to invite this guys parents to our wedding so the parents could meet. We said no and that our marriage was a celebration for us. She was very upset.
Her husband ended up being a huge ass in life. Stole tabs from cars, golf clubs from the airport, etc. Called his wife a @@@@@@@ in front of their kids all the time.
She cheated on her husband for years then he died.
We supported Dawn through her troubles with him.
We never damaged her trust in us with the information she had given us about them. Kept it between us.

During the first few years of our marriage (yes, it was a little rocky), my wife confided some of the details of our issues to her sister. Her sister took that information and shared it with her friends and our friends. It culminated during a happy hour with 2 of my wife's friends (who she hadn't met yet) and Dawn. My wife excused herself to the bathroom. While she was in the bathroom her sister proceeded to tell my wife's friends (whose husbands are also good friends of mine) as much dirt on our first few years as possible. Our friends were in shock and that was it for my wife. She decided she could no longer trust her sister and broke off a lot of the communication they had. It was destroying our social life. Since that episode Dawn has not met any of our friends.

There is SO much more I could write down.

So here's what has happened recently:
She called my wife a year ago yelling and screaming at her about the last 50+ years.
Among the things Dawn said over a 20 minute phone call:
-I don't like your husband
-You're so perfect
-You're so selfish
-You're so arrogant
-You are unhappy, you have not a happy life
(those were all direct quotes)
Mind you, the 2 of them had talked maybe once or twice a year before that for the last 12 years which probably isn't great but my wife has chosen to surround herself with positive people and not negative people. It has really changed her life in a positive way. Dawn talking to my wife like she's the judge and jury on our relationship is just not acceptable. She has been told this but believes sisters are supposed to know about each other.

How do we convince this person to get help? Or are we the problem? Is it our obligation to have her in our life more often? Does she have a mental illness?

Thank you so much for some feedback on this.