If he were still mentally competent, it would be up to him what to do (if anything) about this. Based on what you write, however, he appears to not be mentally competent. If he had previously (while still competent) designated someone his attorney-in-fact or agent by giving them power/authority by a power of attorney (POA), that person (the attorney-in-fact; the one given power by the POA) could sue your sister on your grandfather's behalf for the value of what she took. Assuming that person could prove in court by a "preponderance of the evidence" (more likely than not) that your sister did not own those things but rather stole/took them, a court could order her to return them or repay their value.If your grandfather did not designate someone his attorney-in-fact or agent by a POA while still competent, he cannot do so now--only mentally competent people can create POAs. In this case, you or some other family member could apply to the courts to be appointed his legal guardian or conservator, with the legal power to manage his affairs and over his property and finances. That person could then do as per the above: sue your sister for what she improperly took.If you want to explore the option of asking the court to appoint you his guardian, consult with an elder law attorney.