john truong of renton , washington says sister dropped off 2-year-old boy ronnie tran at his house tuesday .
sister alyssa chang told him the boy was her boyfriend 's son and they wanted to have a date night .
while scanning facebook the next morning , truong read an amber alert issued for the boy and then called police .
truong 's sister was arrested for kidnapping tran and his mother , with the help of the toddler 's grandmother , 65-year-old vien nguyen .
nguyen later turned herself into police for questioning .
the motive for the abduction has not yet been released .