My mum knew a parole officer and once asked if someone hurt a family member(namely one of her children) and she then attacked then, would she still go to prison.
The parole actually said probably not!
They are more lenient on the person, especially as no previous convictions, as the circumstances were uncontrollable and you reacted to something they had done.
In a way you are in a different frame of mind-for some people a mental state-as you witnessed a crime on someone you love.
I think many people would defend a family member, so like I say hopefully the jury and judge will go softly on you.
The solicitor has to tell you the worst fate also. It is best to go in prepared that you may go to prison, as you still may, but if you don't, then I'll doubt you'll care what the solicitor said anyway!
Good luck