Can you give DS his own stuff to do--hard stuff!--to keep him busy while you're helping DD? Or even give him a copy of her assignment and have him work on it alone (and quietly!) while you help DD?
I think it might also be time for a talk with DS about respect for others and being kind. It's a good idea, I think, to teach these HG+ kids that "Why doesn't DD know x when I already know it?" is honest, but not necessarily something that a big sister wants to hear.
...Just like the correct answer to "Does my butt look fat in this?" is ALWAYS "No!" (Or at least "That other one is more flattering to your lovely figure, dear.")
