Quote closetaddict: I also believe that Mariah is Sharon's daughter/Cassie's twin. Mariah said that Helen was a OB/GYN nurse. She probably took her because Ian wanted a child to raise and then later on realized the connection with Nikki(that Sharon was her daughter-in-law). She probably figured at the time that Sharon thought she was having one baby not two.
My thoughts as well. I think she stole her from the hospital. She may have been working there when Sharon delivered the babies.