Possibly; in order for a statement to be admissible it must not be made under any threat of punishment or hope of benefit, and if the person is under arrest and being questioned, then the person must make a knowing and voluntary waiver of their Miranda rights. There is case law indicating that whether a waiver is knowing and voluntary can be effected by severe mental conditions. However, there are many factors that would go into such an argument. Your sister should contact an attorney and make them aware of all the facts so they can know how to best defend her.
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To some extent it might also be helpful in evaluating her particiation in the crime and the punishment if she pleads guilty or is found guilty. Sonding like a broken record, but she really needs to consult an attorney to discuss these issues and how they impact her particular situation.