Yes the twins were buried in Highgate together because Colin did not want them to be apart, i also read Colins partner did go to see the twins , but what the book does not say if the twins coffin was closed or they were viewable, childrens faces are quite small compared to an adult , girls and mothers sometimes there is not much difference but boys and fathers there normally is, it is possible to view children victims of fire, accidents ect, as you can show the faces of children partly, many chapels of rest you can adjust different colours of lighting ect, i believe the twins were viewable and Sheila also, i do know Colin later quiestioned himself by not going to see them, Ralph and June i think their injuries sadly resulted in closed coffins, however you still can sit in a rest room with a loved one even in a closed coffin,just being there helps greatly.