Well, you can draw her something, maybe about something she likes? If you aren"t good and drawing, then you can maybe bake her cookies or other sweets. You don't really have to buy her anything; you can look up projects online and do them for your sister! I wish her a happy birthday!
1) click 'settings'
2) click 'edit profile'
3) in the second box, there's an area that says 'custom user title' in the area box type whatever u want.
4) save and it should appear under ur username.