I'm going to dictate what fish my sister should get
Why are you going to tell her what fish to get?
She asked me for advice on this matter.
It is normal to advice siblings on what to do when you know better but it is not good to tell them what to do. They are people and deserve to make their own decisions and own mistakes. It will help them grow.
She still is a child though.
I understand that she may be young and looking for advice, but at the end of the day you should let her choose for herself. It's her fish, after all!