Together find a jar with a thin neck. Each of you write down thngs you have complained about, about other people in the family. Poke the papers down the neck of the jar, so that you cannot get them back up without breaking the jar. Agree to leave the complaints in the jar for six months. You cannot complain about them in the meantime, either. You can however keep adding to the jar.
At the end of six months you can either break the jar, and see if any of them still matter, or simply hide the jar in the closet and forget about it.