What you are experiencing is all too familiar. Taking care of everybody else gets very taxing on your resources, and burnout can happen. (Depression can be a result of burnout, especially if you have other predisposing factors too). You might want to also visit the caregivers forum on this site. The main thing for you to do is get some help. Make an appointment with your doctor or a counselor. Depression is treatable - you don't have to keep feeling like you do now. Can you find anyone to help you with the kids and your sister so that you can have a break? Being able to get away from those responsibilities, even if only for a little while, will help a lot too. Be sure to keep talking to us here, and let us know how you are doing. You will find lots of caring people here who have been where you are and understand what you are going through. That is a big help too.