Hi JettGirl, and welcome.
DH and I do not have kids, nor will we. However, he has a brother who is disabled and for whom we have assumed much responsibility. His father is deceased and his mother is completely ineffective in dealing with him. Therefore, DH feels responsible b/c he is the oldest sibling.
His brother's disabilities are the result of a failed suicide attempt at 17. He is partially paralyzed, although ambulatory. He has mental issues because the injury (gunshot) effected the center of the brain that deals with judgement and decision making. Therefore, he makes very poor life choices (as if he didn't before). He struggles with alcohol addiction and some other mental health issues (depression, anger, etc). He has experienced repeated homelessness, or when renting, repeatedly has utilities shut off for nonpayment. We've spent countless amounts on groceries (although he does qualify for food stamps, he routinely lets that lapse) and clothing (because he continually "loses" clothes). In fact, we recently bought a small house to provide him with permanent shelter(mostly to relieve our own worry).
While the brother is not technically a child (he is now 31), he has the mentality of a selfish, immature, angry 15-yr old. For whatever reason, he respects and listens to DH, and me by association (he does at least understand that DH is able to help him because I'm on board with it). But there is no doubt we will be watching after him for the rest of his life.
While our situation is different from yours, I do understand having the care and feeding of someone thrust upon you.