Our poor cabin crew have to deal with entitled dicks like this every day. Passengers have gotten worse since Tiger disappeared. If sitting next to your family for a few hours is so critical, just pay the dam $15. Fair enough ask the crew the question, but because you don't like her answer doesn't give you the right to not follow instructions. Do what you're told and complain to the airlines later.
Do I agree with all the rules? Not necessarily. Do Jetstar offer the support and customers service that passengers deserve? No of course they don't. But, if you are given a direction by a crew member and choose not to follow it expect the AFP. If you choose to not following their instructions, your probably going to have a bad Day.