I don't think we've seen the extent of what she went through. She was afraid for her life, but not for her son. She was the problem (in Angela's mind) not her son.
Now that was see what happened to Hannah's entire family over Sophia, it's very clear why Tabitha didn't fight them. It's easy to see outside in and say why didn't she do this or that, but she was afraid and had been controlled and she was able to get out. I think she knew, maybe gut feeling, maybe something else, but she knew they were crazy. She was afraid.