Weep all you want, Hilda! It can help get the frustration out!
I'm not sure if you're mostly upset about dd's reaction what you said, her taking you and dh for granted, or your financial concerns. I think you can chalk the first up to dd's bad day. She was in a bad mood when she heard your comment and so, it set her off. That's all. I think you need to talk to dh, however, about telling her he has to check with you next time.
As for her taking you for granted, I'd like to think that underneath she really appreciates you very much. She's probably too caught up in all the demands of her life to take time to show it. All her dss having special needs and sil away a lot - it must be very hard for her. That doesn't mean she should take advantage of your two, of course, but if she tries, it's most likely out of desperation.
About supporting them - I admit, I don't know how you can let her know you can't help out as much that way. Maybe it's time for her to find a good school program for her kids and get a job. The program may give her boys benefits she can't. Also, it would give her a break from them and their problems, as well as more money in her pocket. I admire her for being able to homeschool 3 (or is it more?) boys w/ special needs. But now it may be time for a change. I don't know if you can say that to her, however, might just start a row.