I'm really surprised. If I told a friend I would be their matron of honour then I would never ditch them for a different wedding, especially not months later. If you accept something like that then you need to honour it, and be reliable. That person is relying on you! A first commitment must always take priority over a second. My best friends mean as much to me as family.
Maybe it's just me; I'm truly surprised. And for a family wedding - again my family must be weird because every single time my brothers have checked with me first if the date suits. Cousin? No, but that's because they don't care if I come or not.