I do not understand why people stick up for their family when they are in the wrong.

When a family member is disrespectful or mean to someone else, why does the other family member get mad when that person defends themselves? I don't care if they are family or not. If someone is in the wrong, I will tell them. I speak my mind, and if a family member was being rude to someone else, I would let the other person defend themselves. It is idiotic to stick up for someone who is in the wrong in my opinion. I just may be too real for my own good.