You can argue with your sister all week and still be first in her corner when someone else starts.

Sibling conflict runs on a different circuit from loyalty. Arguing with somebody every day about the bathroom and the car does not use up whatever makes you stand next to them when an outsider has a go. The two things coexist comfortably and most families know it without discussing it. It is also why the switch can look sudden to people watching.

If somebody starts on your sibling this week, be in the corner before you have thought about the argument you were having. Settle your own business privately, later. They will remember which order you did those in.