There is another way of looking at what you have said.
During a violent fight, I took a broken glass and threw it at my brother,that failed to stop him and he threw a bin at me. So I took a knife jumped over the bin and stabbed him in the stomach.
THAT is the version the prosecution are going to put before the court !
What I find hard to believe is that in the middle of this fight, with your mother still being attacked you did not go and help your her, but decided to start clearing the kitchen by using a knife. I think a jury would share my disbelieve.
My advice is to plead guilty and hope you can get away with a S20 instead of S18. That way you stand a good chance of staying out of jail.