Maybe sit them down and read them the replies you've had to your post. It might bring a bit of perspective; that you and your family are not the only ones going through tough times and that they have some mutual obligations.
When I hear of surly teenagers acting like they are entitled to their own personal servant and the world owes them a living it makes my blood boil. Yeah, I understand they have had a shit time but they are not out on the street or in institutionalized foster care. They can thank you for that.
My son tried this shit when he was 16 so I kicked him out. He'd already started an apprenticeship because I was utterly over fighting with him about going to school. Then the busiess of treating me like rubbish and the house like some sort of hotel started. So, I found him somewhere to board in town and dumped him in there.. Everyone was horrified but his behaviour was making me feel sick and angry all the time. It worked out OK in the end, and even if it hadn't I had little choice. Selfish of me? Probably, but I will not tolerate being fucked around. By anybody. Life is too short. We now have a relationship that is much better than it would have been had I let him guilt me into submission.
You are obliged as a parent to give them support when they are infants--as long as you are able to. You are not obliged to enable bad behaviour.
You have chosen to take on a variety of responsibilities, but you are not responsible for the circumstances that brought you to this point. If they are going to make you feel sick they need a reality slap.