Actually..They do remember Ishiins (somewhat.. anyway)! way back when ichigo encounters Byakuya in SS in the Tower rukia was held... the 13th division captain was with Byakuya and the 13th captain asked something liked "is he the son of..?" or "is he related to...?" and byakuya cuts him off with something like.. "No it cant be" or something of sort.. i forgot what exact episode it is.. but it shows they do remember who Ishiin is.