I guess it depends on their motive in chosing the date, OP. If you think they were deliberately trying to overshadow your wedding (sooner, more exotic etc) then you might have a point, but I'm guessing they'd have booked closer to your date if that was the motive.
My ex SIL tried to do this to us, but she had been doing that sort of thing to XH since they were kids. When we announced our engagement, she announced hers two days later, to a guy she had only known for a few weeks. When we announced our date, she announced her date for just a week before ours. She couldn't get a booking anywhere though (and MIL had gritty words with her about it), so ended up being two months after. The marriage barely lasted a year.
When we announced I was pregnant, they annouced the next day she was engaged to her latest boyfriend. When we announced the due date, they booked their wedding for ten days before it, at the opposite end of the country where no family live, and then complained that we couldn't go. PILs said they wouldn't go either in case the baby came early, and it did - arrived the day before their wedding and she moaned forever after that I had stolen her thunder!
Perhaps your ILs had a limited choice of dates OP, and they had to take this, so unless your SIL is like mine then it may not be deliberate. But as for them getting arsey over you not going, you are definitely NBU.