There is a certain amount of automaticity for joiners to WUS at 11+ for entry at 13+. I don't think that applies from 7/8+, but staff will spot who they think will not make e grade, and the counselling out process will begin.
There isn't much sibling priority, in the sense that all siblings have to pass the exams to the requisite level. I'm not sure if any allowances are made at interview.
Westminster publishes annually the schools from which it's joiners arrive (in the Prospectus, additional information booklet). For 2011, 47 came from WUS (which includes all those who joined WUS at 11+). Schools which sent 5 or more boys: Dulwich Prep, The Hall, Hill House, North Bridge House. 2-5 boys: Trevor-Roberts, Sussex House, Durston House, Fulham Prep, Orley Farm. Plus one each from: Ashdown House, Caldicott, Chafyn Grove School, Cumnor House, Devonshire House, Dragon School, Eaton House The Manor, Habs, King's House, Lochinver House, Lyndhurst House, Mall School, New Beacon, Newton Prep, Notting Hill Prep, Rokeby School, St Anthony's, Sutton Grammar, Summer Fields, St Philip's, The Academy School, Thomas' (Clapham), Thomas' (Battersea), Westminster Catherdral Choir School, Wetherby.
Obviously that's just a one-year snapshot. I should imagine at all of the more academic preps can prepare clever boys to the required level.