When children are looked after by the Local Authority, the LA has a duty to those children to do what is in their best interests. These children lived with you, and their sister, for 5 years, and presumably had a relationship with her which was important to them. It''s reasonable that they would want to maintain that contact.
What are your objections?
If you don''t want to have your daughter''s siblings in your home, the Local Authority should be able to offer alternatives; Social Services may be able to organise and supervise the contact so it can happen at their premises, or at the home of the foster carers with whom your child''s siblings live.
You mention that your daughter has special needs - how severe are these? Is she able to understand who her siblings are, and their relationship to them?
This is a relationship which could be important to her , as well.
If you don''t allow contact, it would potentially be open to your child''s sister to make an application for contact, with the help of an appropriate adult as her litigation friend, and a court would decide whether or not contact was in your daughter''s best interests. However, as a general rule, the starting point is likely to bethat it is in the interests of siblings (including half siblings) to have contact with each other, and this is likely to be particularly true where those children have lived togathr for a long time.
It must be very hard for your step-daughter to have lost her oen parents, andthen to lose her home with you, without also losing her relationship with her sister. Is it possible for you to sit down with Social Setvices and try to come to some arrangements which allows some contact, without putting too much pressure on you?