may be too late with this but the short answer for places you can wait is nowhere! the pickup/drop off bays they have you got no chance of sitting there for 5 mins, you usually get moved on after 1-2mins unless you can be seen to be loading or unloading the car. Driving around the 1 way system until you see your sis is your best bet
as for the m25, stay the f**k away from that bastard! there are a few other ways you can go depending on which way you re coming from, I usually come in from either staines or ashford/sunbury depending which shift im on, but coming from the other side there are a couple of ways as well. Tell your satnav to avoid motorways, you will get traffic anyway you go but nothing like what you will see on the m25