OP made the comment herself about “I am also no spring chicken, 40 next year” herself, as to why she had to choose that date.
I stand by my statement that 6-7 months is a rush job. Not just because of the dress, but because most established suppliers, of cakes, catering, entertainment, cars etc will also be booked up at this point. And venues for that matter! You’ll find suppliers, and venues, but you’ll have to take what’s left in terms of date and supplier, and what’s available off the peg in terms of dress. Plenty of people do it of course, but the majority of weddings are organised more in advance.
I’m glad to see plenty of posters now seeing through OP’s initial half truth post. Clearly her family are very friendly with her brothers wife’s family, and are all invited to each other’s weddings. Those saying they’re not close with their siblings in laws, it doesn’t really matter. The OP’s family are, and that’s where the issue lies.
I do think that if OP’s initial post had read more honestly, about the cross invitations of the families, and the fact that she’s slotting her last minute wedding in the weekend before a long planned wedding her family are already involved in. I don’t think the response would be the same. There are so many other dates available, and since OP has been so vague about the ‘raft of dates’ her brother said he couldn’t do (some people do make plans for over 6 months away), I wonder if in fact there are only a few dates he couldn’t make due to prior commitments, and the one she’s chosen is one of them.