"Small" refers to physical size rather than age. We don't tend to refer to our siblings in this way. It doesn't sound natural to say "my small sister".

You could say "baby sister" if she's only a few months or a year old.  Otherwise you could call her your "little sister" if she's a toddler or a child.

If she's a teenager or adult, you'd call her your "younger sister" ( meaning that she is younger than you are).