Your threads were merged because they are all apart of the same problems and concerns.
I will admit that this is going to be harsh, but that harshness is not directed at you. You and your brother are the innocent parties in all of this.
You may not realize it but your mother has been adding to the abuse. It is emotional and psychological. You don't want to go to the authorities because of why? It might hurt her. She stopped being a victim years ago when she realized the man she married was destroying her children and did nothing to help them or herself other than rely on the 'my childhood was worse' defense. SHE has kept the cycle going and I worry that you may be well on the road of repeating her mistakes. If not you, probably your brother.
Princess, with your fiancé you are like the beaten puppy looking for love and a kind hand when it finds one it will do anything for that person all the while growling and snapping at the world (read the Dog forum about 'fear biters'). That isn't healthy for you or him. He loves and you love him. That is great. However, part of that love should be understanding that you cannot make another person responsible for your own happiness or your actions. It may not seem that way to you right now, but you are giving up too much of yourself for the security that he represents.
You can't continue to live in fear of anyone getting upset with you or what you are afraid they might do. Your fiancé raises his voice and you run off crying. That is emotionally damaging to him as well as yourself. If it hasn't started already, it will, at some point in time, start causing him to walk on eggshells around you for fear of how you will react. He won't feel that he can be open and honest with you or tell you anything negative. It is emotional control over what he thinks and what he does. A relationship cannot survive under that kind of strain. It needs both people working together to be strong and healthy.
Please tell a counselor or anyone (other than your fiancé) about what has happened with your dad. Just because he has slowed down recently does not mean that he won't snap and then it may well be a life and death situation. I don't think you want your fiancé caught up in what might happen.