Usually a beneficiary signs a receipt and release to acknowledge an inheritance and assure the executor and/or trustee that they have no further claims. Sounds like you should consult a lawyer to be sure what you should be doing to protect yourself legally. As for what your obligations are to reach her, that too is a question for a local attorney to determine. You might consider hiring a private investigator to find her. Even if you are not obligated to do so, that might well be the moral obligation. As for as disability, if she lives in Mexico it is unclear what Mexican law would provide in that regard. That means, yes, another lawyer. You'd have to hire a Mexican attorney who could advise you as to what implications an inheritance might have on disability. The facts you submitted are a bit fuzzy. If she is living in Mexico and getting U.S. disability with a U.S. address, that might not all sound cricket.

Another issue. If your sister files a disclaimer (renunciation --- legal papers stating she does not want her inheritance) then it would be distributed under the Living Trust as if she predeceased. You really need to be careful, depending on the legal document and state law, if she cannot be found there might be a risk that her inheritance would escheat (be transferred to) the state instead of the rest of the family. You really need to see a lawyer. Also, disclaimers are pretty technical and tricky and have strict deadlines. Get some professional help sooner rather than later.