It would help if I knew WHICH resistance group, wouldn't it?

YOU KNOW THIS WOULD REALLY BE A GREAT DEAL EASIER IF I WERE CAPABLE OF ANY OF THE THINGS HE'S ACCUSING ME OF PLANNING.

As it stands, he knows someone claiming to be from a resistance group is claiming to have rescued his sister (and a brother he didn't know he had). I'm not sure there's a downside to him knowing that Sirius Black is in this resistance group but if you can think of one, I'm all ears.