Why oh Why does the happiest days of our lives have to upset people?!?.
Sure if your OH's uncle was to offer to pay there wouldn't be a problem...does he think that the meal is only £10?...did your MIL2b not realise that this could happen?, surely she knew about who was getting invited to which part of your wedding?
Why doesn't your OH speak to his mum and see if their budget could stretch to 2 more and ask them to come to the ceremony?...then again this could be a tricky one especially if others try the same thing...
Sorry not much help 