I read somewhere that people who have depression may not necessarily feel it -- or feel it really badly -- all the time. What happens instead is that those people have a harder time handling big changes in their lives, or "triggering events". They can be so simple that most people wouldn't notice them, maybe not even the depressed individual. Or, they can be very devastating/stressful like a death in the family or a car accident or something.

Even if you get along with your family, taking a bunch of kids into your house is going to be a stressful, life-altering event. I have three younger siblings aged 16-7, and I'm also 20, so I know how hard it can be trying to live in a house with people SO much younger than you. If it just happened to you one day and you didn't grow up in that kind of environment, I can see it being that much more stressful.

I think seeking professional help might be a very good idea. Also, have you spoken to a doctor about your depression? If you're on meds maybe they need to be adjusted, or if you're not, meds might help stabilize your mood a little.

Lots of luck.