Miss Boo, people getting married have to have a 'cut-line' or everyone they'd ever met would have to be invited. So you started out on the wrong side of the cut because the happy couple were strong-armed in to inviting cousin Sally that they never see. This was because the father of the bride's brother would be moody otherwise, because his youngest daughter (Sally) is his pride and joy. And she invited them to hers, even though they never went because they barely know her. Luckily Sally has declined as she has tickets to a death metal festival so there is room for you.
So, go if you can afford it, think it will be fun and you'll appreciate being there and genuinely add to their big day. If none of those three apply, give Jon the £200 to add and extra bang* to his stag adventure.
*Sexual slang not intended