The area in front of Domestic Arrivals is a bit of a mess, nothing like the nice clean lines of T5. So she may be difficult to find. You may want to suggest your sister goes up to the mezzanine level, which is a small balcony like floor, quite small and away from the hurly-burly of a cramped terminal. There's a Costa Coffee (I think that's the right brand!) open 24 hours a day there, it's above Boots, and they don't mind people hanging around there. They also accept credit card payments for small amounts without question.