Your sister is at least trying to improve her life financially and help others in the process.
It takes a lot of courage to approach family members and friends about any business. Family members and friends are often harsh, critical and judgmental. It is easy to go to work and exchange hours for dollars. She is at least taking a risk that many others are too scared to take.
There are plenty of people in any business that are very successful because they learned to ignore negative and unsuccessful people. If she is like a lot of people in MLM, she has a desire for time freedom, spending more time with family, helping the communityy, and doing nice things for you. She also simply be sick and tired of being sick and tired.
She is not doing anything illegal. She is working for a legitimate company that sells a legitimate product.
You don't have to like her business or buy her products. However, at least support her.
Now, don't get me wrong. It will take consistency, dealing with rejection, learning new skills and patience to make money. Sometimes people take many months before they start making money.