If your sis and future bro in law are still pissed, tell them that they "should shut the eff up. The next time one of my friends calls you (sis) or mom a bitch, you can and should kick them out of your house. Oh wait, my friends aren't ignorant hillbilly dumbasses, so they won't be calling you (sis) or mom a bitch."
Say exactly what I quoted above to them, except the (sis) part, just motion to your sister at that point.
Gonna win 'em all!