Background: My brother met his girlfriend (now, unfortunately, his wife) in early 2010. She moved in with him that summer. Both she and my brother are perfectionist, OCD type people although she is 10 times worse than my brother. For the first 9 months, she appeared to not want to get to know my family at all. She avoided our family events, limited the amount that my brother had previously entertained at his house and was in general rude at the events she did attend or *allowed* to happen. I chalked it up as a social disorder and tried to give her the benefit of the doubt. /bg
Honestly, at first I thought you might be talking about my SIL, my husband's brother's wife. BIL had a large home and allowed most of the entertaining to go on there. Once my SIL married him though, she put a stop to a lot of it. Many people in the family were incensed because it was "tradition" that he hosted these events, but the problem was that the hosting caused damage to the home, required a lot of clean-up which of course very few offered to help with, often ended with drunk individuals being put up for the night (and then having to clean up their resulting sick), and even had thefts occur from some friends who were guests of his family.
Allow me to offer you a slightly different perspective here. You say that some members of the family regularly show up to potluck style events with little, if any, food but expect their large families to be served. And when they show, they don't adequately supervise their children. While your intentions were good, without pointing out that there wasn't a lot of food to go around, you simply circumvented the issue and started using their kitchen as your own.
It's possible that some, not all, members of your family have been taking advantage of your brother's generosity and he, until he was validated by his girlfriend, put up with the behavior because after all, it's family, and that's what families do. I'd be willing to bet he's been holding back years worth of resentments but putting up with them because he thought he had to. Now that GF is in the picture, she is telling him that no, showing up to a potluck style party with your spouse, five children, and no food, is not acceptable. Allowing family to destroy your home because they refuse to adequately supervise their off spring is not acceptable either. He probably feels empowered for the first time, in a long time, in setting acceptable limits.
To be fair, I really don't think what you did was that bad and you may have gotten the brunt of some of the residual ill-feelings he has for others who have long taken advantage of him. However, I do think that you need to re-evaluate how you feel about his GF. She has upset the normal "status quo" but that does not mean she is evil or controlling. Since the two of them have effectively formed a partnership, it's understandable that the game would change. Everyone needs to accept this and move on from there. Not insist that everything proceed as usual.