I will first explain to you that you can NEVER just "get' someone to like you. That isn't how it works, unfortunately.
First off, you need to ask yourself a question. Why would you want this girl's approval in the first place? If she isn't willing to accept you, then that is her problem. Her brother should realize that.
Some steps her TRY to get her approval - be yourself. Prove to her that you are in no way a "priss". Try to be openly kind to her, etc.
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.