People think my little sister is cute, but I don't think so.
Why don't you think your little sister is cute?
Several reasons. She smells, she's ugly and she's a pain in the ass
It is wrong to make fun of your little sister when she can't take care of herself. You were like her once and people probably felt the same way about you, you should be nice.
She needs to be aware of what other people think of her before she gets older.
If you want her to change, you'll probably have to intervene and help her change. If you're not comfortable with that, talk to you parents about your concerns. She may be too young right now to understand what your concerns are.