This ↑↑. There is either responsibility with authority, or no responsibility with no authority. Unfortunately, like most SMs, you have been put in the place of having much responsibility, but with no authority. In other words, you wind up finding yourself in the role of a servant to the family vs. what your true role should be—DH’s wife or SO.

So, SM finds herself at the bottom of everyone else’s priority list, while everyone else is to be at the top, including BM. It’d be one thing, like you say, if you even got a thanks now and then or someone putting at least some effort into doing their share, such as SD and boyfriend cleaning up after themselves. But, no, instead you get blamed and punished for not making everyone happy.

You are getting a raw deal, and you know it, and that is why you are upset. But, again, like most SMs, I’m sure DH, SKs, BM are all acting like they are the ones getting screwed and you should be doing more. Yep, you should be doing more, so in the future, they can all treat you like DH’s sloppy seconds whenever they are all around vs. DH’s wife. That will be your “reward.”

Some would say it is DH’s fault for not putting his foot down and even your fault for not just saying “No,” and, in many ways, they are right. It is your and DH’s home, and I would strongly advise that you let him be 99.9% responsible for the care of his 19-year-old daughter. If you continue to cook and clean for her, that is sending her the message that: Yes, you are her servant, and all with DH’s blessing. Stop doing this. You are waiting on these ingratiates and not even getting paid. No one would ever, ever be expected to do that, except for a SM, of course.

Society has it all wrong, as your post proves. As a whole, SMs go to great lengths to try to keep everything together for themselves and “the family.” Her payment = anytime any little thing goes wrong she is to blame and if she dares to contest that, she is Evil SM. I say, fine, be evil SM all you want. At the end of the day, you’ll feel better about yourself and your marriage to DH will perhaps still be intact. You’d be surprised how quickly when SM says, NO, and means it, DH suddenly starts getting the balls to also start saying NO, as in telling DD that if she and her boyfriend can’t start cleaning up after themselves, then boyfriend isn’t welcome any more, BECAUSE now it will be DH cleaning up after them rather than you! And, don't hesitate to let their dirty dishes sit in the sink for weeks on end, even. They are the ones who need to get the message, not you.