Wedding Date Schedule Conflicts

Spent 2mos deliberating w/ hubby's family, before having our wedding at a date & location we didn't want, to accommodate them. On xmas eve sister-in-law informed us she'll be holding her wedding on my 30th bday. Got engaged 1mo ago and already put a down pmt on the venue. No one consulted us about it. My SIL was at my bday this year when I explained I was keeping it low key b/c we were planning on doing a big trip to Vegas for my 30th--possible b/c it falls on a long weekend. A long weekend is convenient for a wedding as well, but I feel extremely disrespected that no one would even think to talk to us about it. Gave up the wedding I wanted to accommodate this family. Will now lose the 30th bday I've planned on for yrs b/c they didn't even consider me. How do I bring this up? Is it ok not to attend? To ask them to change the date? Would cover new down pmt. Feel stuck in a position where now I'm the diva for even considering, after bending over backwards for them before.