Start an SIP: Systematic Investment Plan (SIP) in a mutual fund is a great gift for your sister. You can invest a fixed sum on a regular basis for your sister for as many years as you want. You can choose to invest via SIP in an equity or a debt fund, depending on your risk appetite. You must consult your financial advisor if you have limited or no knowledge of mutual funds.
Make sure you discuss with your financial planner if you have little knowledge of any of these financial products.