I can understand why you were upset. My mom lives alone and her brother helps her with stuff like that a lot, which I'm sure is a godsend for her. I can't imagine trying to do it while also caring for 3 kids!
That said, do your brother and his girlfriend have something big going on in their life right now? I ask because we recently moved, and for the 2 months prior to the move, every weekend was precious to us if we were going to be packed and ready by moving day. Well my MIL sold her deceased parents' house right around the same time, and there were a few Saturdays when she asked family to come help clear it out so that it could be ready for the new people. Totally understandable, it needed to be done, but she had my FIL and BIL helping too, so it did peeve me a bit that she also asked my DH to come help when she knew that we were working our butts off trying to get our own home ready.
That said, I had the good sense to accept my DH's decision to go help, and I wouldn't have dreamed of pestering him with increasingly angry phone calls while he was there -- that wasn't nice of your brother's GF.
DH+Me 1994 DS 2004 DD 2008 DDog 2014