Hi Sophia
I found certain tips that may help against depression.
However, some of the following tips seem to help me.
1. Get into the routine
2. Set Goals (many people that experience depression feels like can't accomplish anything)
3. Exercise (this may temporarily boost feel-good chemicals called endorphins)
4. Eat Healthy (in case, depression makes you overeat)
5. Get enough sleep (avoid naps and try to go to sleep at the same time every day)
6. Take on responsibilities
7. Challenge negative thoughts (this is the main point that you should try to change)
8. Discuss with a healthcare provider before using any supplements (including fish oil, folic acid, and others)
9. Do something new (push yourself to do something different such as read a book, go to a museum, and others)
10. You should try to get fun
11. Do not use alcohol and other drugs (Substance misuse is common in those who suffer from depression. In any case, these substances may worsen your mental health.)
I think you should tell your sister to try these tips if there are no other methods to treat her depression. Good Luck!