No. If there's a seat free, I will take it. I am not going to be fretting about who may be potentially getting on further down the line. If someone elderly or disabled turned up, then of course I am going to give up my seat. But standing so a parent or teenage child can sit? I don't think so.
A few years ago, I remember a family of 4 (2 teenagers plus mum and dad) walking the entire length of a 12 carriage train looking for a "4 together" set of seats (which, as this was a rush hour train and folks were already standing, was extremely unlikely), and the father thundered - "People are SO selfish, they won't even let a family sit together" - I wanted to say "So you expect 4 complete strangers to stand all the way to Stevenage just so you can sit together?". Later, they got excessed for travelling on off peak tickets in the middle of the evening rush, which I am sure probably meant they never took the train again.