"Ok so I don't see them that often" I can't imagjne why 
You sound really bloody hard work.
"I'm surprised they've invited you at all." Me too!
"It's just we are a small family." Completely irrelevant
"We live too far away to just go for the evening." They don't really want you there at all then, I'm thinking you MUST have done something to offend them at some point.
"The same bride had an affair with my nephews friend yet he still goes ahead with the wedding" ffs! The number of bm's, how big the wedding is, THEIR PERSONAL relationship - all NONE OF YOUR BUSINESS!
Apologise to them and your sister, ASAP! decline invite send card and small gift.
"has you sister and mother been invited to the whole thing?" Erm thinking the sister of the op is grooms mother? So higher priority.
I am not buying for ONE SECOND that you've said NOTHING to anyone about your clear disapproval hatred for the bride!
"An aunt coming from Australia is only invited to the evening do" no I'm calling this out as a lie too aimed at eliciting a change in response here.
Unless it’s a tiny wedding or there is a specific reason - yea I'm guessing the "specific reason" is how op treats the BRIDE - why anyone would want someone at their wedding who is going to look down their nose and quite possibly make disparaging comments about the bride is beyond me!
also - bear in mind this is how the op (presumably) behaves sober! Add alcohol to this fraught situation it's a potential nightmare!
Re bridesmaids op hasn't said who they are - could even be brides sisters!
"Your nephew is weak"
A - how do you know the nephew isn't in total agreement - may even be
his idea! Especially as he knows you better!
B giving in to the aunt if that isn't what he or the bride would be weak!
Again! Not buying that YOU not going means your parents "can't"
And instead of admitting what you said to cause a rift (which I have no doubt is what happened) you're blaming EVERYONE else and flouncing - mature!
"My husband and I got married abroad to avoid all this" oh my god you are SUCH a hypocrite!