No matter if the fire brigade have to cut you out of your house to attend, no matter if your day job is as the tattooed lady in the local circus, your family/sister etc shouldn't exclude you from a wedding so cruelly.
What does your instincts say? I'm guessing NOT to go.
I wouldn't go.
It's not about 'ah but she's your sister' someone tell your family that!
It's not about 'making things worse if you don't go' how is being excluded from a wedding any better than taking the hint that you're not wanted/needed/thought of.
I say the only chance you have of making a point is to say, I am not playing your games any more, either treat me with some respect, or leave me the heck alone.
My 'mum' moved house, excluded me from pretty much all of it, and even when I expressed my bewilderment at her awful behaviour, it's all la-la-la and i'm supposed to just carry on as normal. This was the tip of the iceberg, she's the last member of my family I spoke to, as I won't put up with being treated like crap anymore.
Set your boundaries and bugger the lot of them.