I’m not sure what kind of help you’re looking for. There must be something really, really sweet about your partner to keep you hanging in. You can’t complain about her not getting a job when you’ve so consistently picked up the slack and let her maintain her fiction that she is working on commission. Rather than try to make her be different, you need to look at what you can do differently.
What concerns me most is that you’ve given up having your own son living with you and have accepted the sick sister into the household with no change in the financial arrangements. From your son’s point of view, he’s been replaced by the sister of someone he doesn’t even like. This is likely to have far-reaching consequences to your relationship with him. Is the relationship with the girlfriend really worth that?
Given what little I know, I’d suggest you take a huge step back. You should try moving out. And then establish separate households. Date your girlfriend if you like but don’t support her. Reestablish your relationship on the basis of more equality. She will probably accuse you of being horrible and try to make you feel guilty. If you need to feel guilty (which I don’t think you do but apparently it is a tactic that works with you), feel terrible for underestimating her ability to be a fully functioning adult and for putting her before your own kids. Then please try to be kinder to yourself and do what you need to do to make some important changes.
I wish you well.