iona costello , 51 , and her daughter emily , 14 , missing since late march .
pair from posh suburb of greenport , long island , often went to shows .
video shows them with suitcases , but relatives began to worry after daughter began missing school .
mother told workers at her horse farm that she 'd be ` back on tuesday '
relative said that mrs costello had been ` under a lot of stress ' during the legal battle over her husband 's estate .