blanca cousins fell to her death at exclusive address in hong kong .
briton nick cousins , 57 , and teen 's filipino mother , grace , arrested .
mr cousins said he and his family are ` distraught ' after their loss .
they may yet lose custody of their second child , carla , aged 14 .
blanca and her 14-year-old sister 's births were allegedly not registered .
teens did n't have passports and were educated at private tuition centre .
police have said that blanca may have been ` unhappy with her life '