["My sister in law irritates me to no end.  I wish my husband would talk to her, but in their family the rule of law is don't make waves.  It's ridiculous."]
I'm sorry to hear that, hopefully she naturally mellows out.
My sister in law irritates me to no end.  She lives with my mother in law and cannot take care of her kids at all.  My MIL basically raises them, feeds them, houses them, clothes them, etc.