Welcome to SR, and I am so very sorry for your situation.
You NEED to report your brother's behavior to the authorities - it will only get worse and you need to think of this as self-preservation, for you and your mom. Just because he is drunk at the time of the abuse doesn't make it OK, and it's not fair to your 60 year-old mother (and you, your fiance, anyone) to be subjected to threats or live in fear.
Stay strong! You will find lots of support here!
Jo Ann