Hello
It sounds like you miss your mum, nan and brothers very much. Your parents splitting up is about how they feel about each other and there may be feelings of loss, disappointment, anger and blame for everyone involved. It seems that your brother is very angry with your dad. You need support from both your mum and dad and should not be asked to choose between them.
You have done the right thing to tell me what is happening. I feel concerned that your father is frightening you by saying you will need to move out if you speak with mum. I wonder whether there is someone you know who could help you to tell dad how upset you feel? How would it be to write down how you feel and give it to dad or someone you trust? If you'd like to speak more about how you're feeling and what you could do, you could chat to a ChildLine counsellor by calling 0800 1111 or by logging in for an online 1-2-1 chat .
Take care
Sam