Hi Lost and found,
What I see is your sister trying to create a Karpman Triangle... .actually I see this as more than one triangle... .
Triangle 1
1) Your sister sees herself as the victim (not receiving inheritance/not able to care for mom)
2) She sees your brother persecutor (he gets to take care of mom and he is taking her (sister's) inheritance)
3) She sees the family as her rescuers (she pulls them in to take sides with her and push your brother out)
or
Triangle 2
1) Sister see's mom as the brainwashed victim (she would never cut sister out of inheritance if she were well)
2) Sister see's brother as persecutor (he is keeping mom under his control)
3) Sister see's herself as the rescuer (she will bring the family together to rescue mom from your brother)
or
Triangle 3
1) Sister is the persecutor (going after your brother)
2) Your brother is the victim (falsely accused by your sister)
3) Family is the rescuer (rescuing mom from brother and backing up your sister)
or
Triangle 4
1) Sister & Family are the persecutor (going after your brother)
2) Your brother is the victim (falsely accused by your sister)
3) You are the rescuer (backing up your brother)
Below is some information on Triangulation and a link to information on the Karpman Triangle that you might find helpful in determining how you can react to your part of the triangle.
Brother takes Sis's outbursts in stride.
I think he is on to something here

Me, not so much. I am disturbed that these emails bad mouth my brother. I am upset by hearing from her at all (not sure how she got my new email address, but she did).
Are you accepting a role in the Karpman Triangle? Can you step out of the triangle? What options do you think you might have?
TriangulationWhen two people are in some conflict and one enlists or aligns with a third party to support their position. Triangulation, as coined by Murray Bowen MD is the “process whereby a two-party relationship that is experiencing great intensity will naturally involve a third party to reduce anxiety” (Bobes & Rothman, 2002).
This unhealthy dynamic commonly happens in family, close friendship, or organizations. Who or what is right is determined more by the pairing than the issues.
Karpman triangleThe Karpman Triangle, described by Stephen Karpman is a very useful tool for understanding "stuck" relationship dynamics. The roles are Persecutor, Victim, and Rescuer. We may start in one position, but as another (or others) shift around the triangle, so do we.
https://bpdfamily.com/message_board/index.php?topic=108440.0Just some food for thought.
Take Care,
Panda39