You're over 18. You don't have to stay.
Your 23 year old brother may be depressed. Of course there's no way of knowing from what you've written, and we don't give out medical advice here, anyway. But he has made his own bed. As have your foster parents.
Find roommates to help keep your costs down, move out, and save money. Not for your foster parents' retirement, but for yourself. You are entitled to a future. You don't have to be yoked to them and their economic choices.
If you want to be helpful, then I'm not suggesting you be mean or let everyone starve. Rather, I am suggesting that this is your money, and that needs to be crystal clear. If you donate or lend (good luck getting it back-- operate under the assumption that any money you "lend" is inevitably just going to be a gift) anything, then it's on your own terms.
And that also means that you are under no obligation to continue. A one-time gift, if you choose to do that, is just that-- one
time. You don't have to repeat it.
BTW, once you move out, it's highly likely that your foster parents will either find a renter or work or otherwise do what they can to make up the shortfall. They're doing nothing because you're enabling the situation.
Leave. It will be better for both you and them.