Not experience myself but I know cases that olders get into St Paul's / QE / HBS but the younger silbing didn't get in last year or this year, with similar ability and similar prep level as the olders.
It is just the reality that the London 11+ is a different level game compared to 3-5 years ago.
To ease the pressure of the young sibling I think it is important to manage the expectation - first foremost the parents expectation, if that goes away younger sibling shouldn't feel the pressure as much. Sometimes positive input from the older sibling and some fun activities distraction help too (depends).
I once mentioned it is like the Harry Potter sorting hat, if you TRY HARD and not get to the school you want it is probably not the right school for you. On the positive side, the well prep DCs will BE sure thriving at the less competitive school since during the process they've developed a lot of discipline, knowledge acquire ability and sense of competition and resilience is a bit part of that..(at the minimum, having fun doing NVR or maths puzzles).