I don't see any issues with jumpseating while your family is on revenue tickets inthe back and nobody would care.

In your scenario, there's only one thing you don't want to do. Don't buy yourself a refundable ticket as a "back-up" with the intention of getting it refunded if you can make it on the jumpseat. Double dipping by buying a refundable revenue ticket when your primary plan is to non-rev/jumpseat is considered a no-no.