Disinheriting a child

Dear Len & Rosie,

My elderly mother recently passed. I am trustee and only beneficiary of her trust. I have a sister, who I have not seen or heard from in 40 years when she said she hated all of us and never wanted anything to do with us again. She even gave up her five-year-old son for strangers to adopt and would not let my parents adopt him. In the last few years she has visited my mother a few times a year. Now she has retained a lawyer to challenge the trust. Her court petition states that my mother was insane and that I committed elder abuse by forcing her into making this trust, but my mother left her out of the trust because of what she did in regards to their only grandchild and how she treated her and my father.