iona costello , 51 , and daughter emily were last seen on march 30 .
were heading into manhattan to visit the theater when they disappeared .
mom 's stepson , george costello jr , was visiting the hamptons at the time .
has a long criminal record and has a tense relationship with the family .
he returned to florida two days after they went missing and was jailed .
police have so far said they do not suspect foul play is involved .