Trinity explained it perfectly. I just wanted to add that I would ask your mother in law to share test results with you and your husband to make sure she is cured. Since she hid it all this time I would want to make sure she really did treat it or treating it.
You mother-in-law has stated she is treating for hepc and is currently undetected for the hepc virus. She can't infect anyone if no virus is present in her blood and your family is in no danger. The hepc virus is not passed from person to person like other viruses. It is a blood borne disease and blood must enter the bloodstream of the other person in order for infection to occur so regardless of how susceptible your daughter is to viruses your mother-in-law poses no threat at this time. Many of us have had hepc for years and no other family members are infected so everyday interaction doesn't put family members at risk. You shouldn't share personal items such as toothbrushes, nail clippers, etc as to ensure there is no blood contact, however, the odds of transmitting the virus in that way is very low. If your mother-in-law achieves a sustained virological response which mean no virus in her blood 6 months after completion of treatment she is cured.
Trinity