Chinchillas can be slow to trust, so as you said your sister was teasing her then it will take time once that stops for the chin to trust it's not going to happen again. So long as you spend time (at least an hour a day) with her she will learn you aren't a threat. You don't say how the chin was teased, but any negative treatment can take awhile to get over, months to years depending on the treatment. You say you had her for two years, but not how old she is, did she have a previous home where she could have also been mistreated before you got her? Also chins personalities play a large role in how they act too, some chins are cuddly and enjoy crawling all over their humans, where as some just don't like to be held at all and everything in between. Most chins actually don't like being held, they mostly like to hop around and on you rather then be held by you. Unfortunately youtube is a bad example of how chinchillas are, you get a very skewed view of chinchillas since people are only going to primarily post their outgoing and more friendly chins.
Mostly though it takes time and patients to earn a chinchilla's trust and love. They are also highly social animals so you really do need to spend a lot of time with them everyday to build and maintain a friendship, it's not something that happens overnight and they will get mad if you don't spend time with them.
If you do a search on the forum here you should find a lot of bonding techniques, like reading aloud to the chin, singing to her, talking to her, handing her treats (or hand feed food or hay) and twigs/sticks to chew, and just sitting with her. You generally need to spend time showing you aren't a threat and aren't going to just grab her or pester her every time she sees you or your sister. Also you have to go at the chin's pace, there is no way to fast track the bonding process.