How do I deal with my fear?

BIL threatened me and my family. The threat was vague but vicious. Something along the lines of cause the greatest pain, ruin everything we work for etc. etc.. The issue stems from $ and MIL. He has a summer home, we do not and work too hard to be able to enjoy one.... I am the sole caregiver for his mother. His family has been through a lot. All his children have some sort of issue...either cancer, or seizures, or any number of unexplained illnesses. My husband and I made it our mission, in the beginning, to try to make their life better. We would always pay for the outings together with them and choose to have our family do without so we could be charitable to them. We feel blessed that our children don't suffer as theirs do. So we would do what we can for them. Not expecting anything in return. Till I found out, accidentally, they own 2 mansions free and clear. My house is so small the vacuum cord reaches every room from one wall plug. I tried to tell myself that they have their reasons but the lie doesn't seem to be isolated. I've been taking care of MIL for years now. He manipulated the bank into putting him joint on her bank account. Not sure how the bank did it with out her signature (I'm POA) but it happened. She refused to sign the paperwork because she didn't want anything on the account to change. I questioned "CHASE" why the document said, signature on file he didn't know, it's not proper procedure, but he couldn't correct it.... When we moved the $ to a better deal account. BIL threatened us again. The last time they took MIL away she came back hurt from a fall. (no one told me she got hurt I found the bruises during bath time) I am trying a new approach with MIL trying to get her interested in the outdoors so she can get vitamin "D" naturally. She doesn't participate but she likes to watch me work in the garden. One of the plants we picked I had germinating in the house is poisonous...I didn't know. There was no warning on the seed packet....we are really not in the habit of eating seeds or decorative plants, the thought of something poisonous never crossed my mind... Well, BIL went crazy just falling sort of accusing me of plotting to poison MIL. His ranting seems strange...like he's setting up a strategy. He is making up elaborate excuses for strange things... mind you this is the guy who wrote us off... no civil words exchanged face to face in years. I'm scared, now they want to take MIL again. I want her to be happy and to spend time with other people she loves. I could use the break. But I'm SCARED for MIL and us.
I cope mind you...
But I get physically ill every time I hear from him. Like woman's intuition on steroids.