Can my mother who is a corporate lawyer write testimonies for my sisters to say against me?

Unfortunately you are allowed to write whatever testimony you want for people. Whether the person will actually testify to that is a completely seperate matter. If your mother is an attorney, my speculation is that what she is having your sister sign and execute is actually an affidavit, which is a sworn statement. These statements can be used to support procedural maneuvers like motions for summary judgment or, more likely because you mentioned adoption, as supporting documents to the probate judge to allow for full adoption.   It is HIGHLY unusual for a parent to lose custody to another family member. I can count on one hand the times that I've seen it happen (and one in which I represented the aunt and uncle in successfuly getting custody). The number one reason for a non-parent succeeding is that the other side just gives up.