That may be seen as evil involving sister, say (7)
I believe the answer is:
visible
'that may be seen' is the definition.
(that which can be seen)
'evil involving sister say' is the wordplay.
'evil' becomes 'vile' (I've seen this in another clue).
'involving' indicates putting letters inside.
'sister say' becomes 'sib' (short for sibling).
'vile' going around 'sib' is 'VISIBLE'.
'as' acts as a link.
(Other definitions for visible that I've seen before include "Able to be seen" , "Showing up" , "In view" , "Apparent" , "In sight" .)