I don't think, currently, Melinda has a husband. And she just might have had an affair, resulting in pregnancy, hubby dumped her, and it could be why she then asked her parents to take Haley.
However, that is all speculation, and until we learn more of Haley's history, the only thing we know about Haley, is that her sister went to the U.S. when Haley was a baby. Then her parents died when she was very young, leaving her with an envelope with money, and instructions on who to contact to get her to the U.S. and Melinda. She has evidently lived with her some time, as she mentioned to Melinda that there were neighbors, friends, teachers, records, who would have information. That her living with Melinda, who was paying for her education, was not a secret everywhere.