In 20:16 Avimelech tells Sarah, "I gave a thousand pieces of silver to your brother.."
Why did he not say "to your husband"? he already found out about the deception and that is why he is sending them away? Why keep up the charades? He knows, they know he knows, so why "your brother"?
One could say it shows the importance of keeping up social niceties. Sure he knew. They knew he knew. He is even throwing them out of his land over this incident. But you don't call out a person directly.
They told him they are brother and sister, it is impolite to say otherwise, even though he knew the truth.
Anybody got anything better?