Needed to fly when staying with Dad and sisters on vacation (8,4)
I believe the answer is:
boarding pass
'needed to fly' is the definition.
(a boarding pass is needed to board a plane)
'staying with dad and sisters on vacation' is the wordplay.
'staying with' becomes 'boarding' (to board is to stay in a certain place. I am not sure about the 'with' bit.).
'dad' becomes 'pa' (synonyms).
'and' says to put letters next to each other.
'on vacation' means to remove the middle letters.
'sisters' with its middle taken out is 'ss'.
'boarding'+'pa'+'ss'='BOARDING PASS'
'when' acts as a link.
(Other definitions for boarding pass that I've seen before include "prerequisite for high-flyers?" , "travel document" .)