Just because your sister has issues and, I assume, taking meds does not (a) mean you should not take meds, and (b) does not make you your sister!

If you are experiencing waves of depression throughout the course of a day, you may be experiencing bipolar rapid cycling. This does not typically resolve itself without appropriate therapy, including medication. You may want to consider meeting with a psychiatrist just to get their take on things. It could become clear what it is you are dealing with and what options are available. Arming yourself with knowledge is so much better than not.

I'm sorry for what your are going through!