So what is the probability you sat next to either your mum or your brother's girlfriend given they were sitting next to each other in a 10,000 seat arena?
If so they are in a pair and you can sit either side of them. If this is true it acts as there is one less seat in the arena. Do you follow?
Now are there any rows that end? I.e. can one of these two people be on the end of a row, i.e. you will not be able to sit on one side of the pair?