Getting married month before bride's cousin

Hi everyone. So this has caused quite a bit of drama for my wife to be. We got engaged a few months ago and decided we want to get married next year. We decided on April and paid deposit but when we shared news with girlfriends brother he said it would be extremely difficulr ro get time off work as its his busiest month of year. We panicked at first but then decided we'd change dates. The only time we could change it to was August as place was fully booked until then (very popular venue). It had to be either midterm April or summer as a lot of my siblings are teachers so only time they could attend as they live abroad (Australia, middle east etc ). Luckily they were able to change the dates without charging any extra fee, however it was a month before gf's cousin. I also should add it is only our immediate families going to wedding. We are having a party when we get home for all our friends and extended family but its a low key event in local pub, probably a bbq. That will be two weeks before hers. My gf's cousin has not had the reaction we were hoping for, either has her mother. She said we should have asked, it's bad manners, that they planned theirs for ages and we cut in front. Are we being rude here? Never thought it would be an issue until their reaction.