Will a sister who is mentally ill be held accountable for her lying and stealing that she committed before her illness?

A person is accountable for their actions during their sanity. If your sister did not repent and return what she stole before her insanity, then she is held accountable for that. You must return the stolen items to their owners from your sister's money, if available, or seek their forgiveness. Repentance for human rights is not accepted except by doing so.