Although this article dates back 4 years from today, nothing has really changed. The situation around foster carers being available to look after siblings still remains a challenge across the UK. We at Sunbeam prioritize recruiting prospective carers who could provide siblings placement and put our best efforts to try and keep siblings together at all times.
http://www.communitycare.co.uk/2013/05/10/foster-children-split-from-siblings-and-moved-too-often-due-to-carer-shortage/