If a child is a close contact of a positive case in your school, it is a legal requirement that they be asked to self-isolate and stay at home. This means they must not leave the house, except to go in the garden and they are not allowed to meet anyone from another household. No other members of the household are required to isolate unless they, or the child, develops symptoms. If the child doesn't have symptoms, other siblings can still go to school.
There is high anxiety in many school communities, however, about the current rate of infection, particularly for those who have had high instances of Covid cases. Parents may want to consider keeping siblings at home in this instance, as a way of helping to reduce the risk of spread of the virus within the school community. Whilst this is not official guidance, the LA are supportive of schools and parents in these instances.