Thank you for you letter, it’s a really great idea to get some advice when things are difficult. From what you say things at home are very difficult at the moment and your dad is behaving in a very unsafe and bullying way. This bullying is actually a type of abuse. It seems you feel that you are unable to move away from your dad because of your family not having enough money. A lot of families feel that they have to live with abuse but there is help available. You, your mum, and your sister have the right be safe from abuse.
It's concerning that your dad is abusive physically, mentally and verbally. Sharing your opinions is a positive quality and you should not be hurt or abused because of this. You seem like a very strong and caring young person and it’s really positive that you want to do something about your dad being abusive towards your family.
You have said that your dad has drained all of the power out of your mum but it does seem like she still has some strength to protect you and your sister. You describe how she does everything for you and sticks up for you. I'm wondering how your mum would feel about the possibility of taking action to protect herself, you, and your sister.
This can seem like a really big and difficult step to take but she may find organisatons like Women’s Aid helpful. Women’s Aid is an organisation aimed at supporting women and their children to be safe from domestic abuse. They understand how difficult it can for people to separate from abusive partners. Your mum may find it helpful to talk to them about your situation and to discuss the choices she has to protect herself, you, and your sister.
If you think your mum is unable to do this, you could have a look at the young people's part of the Women's Aid Website called The Hideout. It has lots of information about how young people can be safe from domestic abuse and what they can do to stop it. The Who can I talk to page has a list of people that can help you at the moment and some information about what they can do.
You may also find it helpful to talk to ChildLine by calling 0800 1111 or by having a 1-2-1 chat online. Perhaps talking some more about what is happening and the choices you have to be safe could help you to decide what to do. The ChildLine message boards can also be really useful. You can talk to other young people in similar situations to you and find out what helped them.
If you feel unsafe at home at anytime or your dad has injured you, your sister or mum you can talk directly to the police and it’s also really important that you get emergency medical help if needed by calling 999.
Hope this helps. Take care,