No, that would be fine. The money would remain yours, and your sister would be acting as a fiduciary on your behalf. This should not affect her eligibility for SSI or any other public benefit program.
Harry S. Margolis practices elder law, estate, and special needs planning in Boston and Wellesley, Massachusetts. He is the founder of ElderLawAnswers.com and answers consumer questions about estate planning issues here and at AskHarry.info.