If you’re sure about this incident then you must tell your brother and ask him to file a case against his wife. You must have proof against her if you’re asking for legal support. Be more careful and stay alert at home and make sure your mother is safe and protected. Oppose your brother’s wife and accuse her of stepping up to threaten your mother. 
Make sure all the proofs are in your favour and your brother’s wife is not supported by anyone. Hire a lawyer and with his/her help ask for justice for your mother.