If you have booked "impossible" tickets, you should plan on UA cancelling one or both of them at some point. Therefore, if the two tickets are "impossible" and both on UA, you will need to cancel your family ticket. As to consequences for your family, there are no adverse consequences for them if you no show. Every passenger has his own ticket. The fact that the tickets are booked into one PNR is irrelevant. On the other hand, if you are not with them, they won't get your benefits.

Of all the "benefits" all that really likely matters is the baggage allowance. How many bags and is it worth rearranging their travel or yours merely to save a bit? You will have to do the arithmetic.