I feel for you OP. Your SIL sounds like a bit of a bitch, the comments she's made to you are terrible (who actually says "benefits scum"? You work full time!) and the fact that your DM has said she was surprised to receive the save the dates 1) because DB had told her they didn't have a venue booked and 2) because she didn't think they were marrying so soon!
You've since found out that the venue is her parent's garden, available all year and the church is the one she was named after (am I the only person that thinks it's odd to name their child after a church?) in her little village is only available for one Saturday next year (doesn't ring true, but whatever).
I'd be pissed. My sister got married on my grandfather's birthday a couple of years after he passed. He was an incredibly important man to both of us and we miss him dearly. Her marriage lasted 2 years. My grandfather's birthday is now overshadowed with this fact. It's sad, but harder for her. Don't let them overshadow your birthday, as others have said, celebrate without them, or do it the week before and make a fuss of their upcoming nuptials, there's nothing like fighting bitchiness with love (hard as it is).
I'm getting married the weekend after my FILtb 75th birthday. We talked to him about it, he's not one to celebrate things so doesn't care, in fact, I think he's quite pleased the family will all be together for once!
Good luck sorting it OP, but I you're going to need to suck it up, perhaps get a big badge/helium balloon saying x0 today and let everyone buy you drinks (I'm a big celebrator of birthdays and I'm 30cough) . Perhaps your DB and SIL will allow you to invite your bestie, not all your friends, but someone you'd really like to be with on your birthday. 