You can fight with your siblings all week. Nobody outside gets to.

Fighting constantly and defending each other instantly are not a contradiction, they are one arrangement seen from two sides. Inside the house you are equals and everything is up for argument. Outside it, somebody going after them is going after the household you both belong to, and the loyalty switches on before you have thought about it. That reflex is one of the more reliable things you will ever have.

Argue at home and close ranks in public. Do not hand anybody outside your complaints about them as material. When it is genuinely bad for them, be the first call instead of the last.