Teeth Bleaching

i was just wondering how do teeth become mottled and if they are how can youu get rid of it without the need of bleahing or veeners, my little sister has mottled teeth, and she is gettin them bleched it about 2 years time she is 14 now, and she is getting braces soon so once she has her braces off she is havin them bleached but she wants to no how to decrease the affect of mottled teeth without bleeching or veeners, as she is concisous about it.
I also want to ask from like my point, is there any chance of me gettin mottled teeth coz my little sister has im 17 now, and i was wondering how do teeth become mottled and how to prevent it?

So if youu can answer those questions thanks

frm Flo and Iona x

Do veneers or crowns