Sounds like you are more of a parent to her than he is, and he is acting like a spoiled teenager. If he isn't willing to step up and be the parent he should stay out of your way. She was mowing when you got home, so it sounds like she was finally doing her chores. He should not undermine you. If he won't back you, he can do all the chores himself. 

Getting off topic with this, but is your insurer OK with this? “ I carry health and dental on all of them since my insurance is better than their biological parents.”  Having worked for a health ins. carrier, at least for the org. I worked for, only legal parents (usually meaning either bio-dad or BM or adoptive/ foster parents) can carry health ins. on their children.  In my state, there was even a big to-do about this very deal—CODs having multiple or technically, if not illegally, inappropriate health coverage.  My DH, for instance, had to provide “proof,” which included a copy of the divorce decree pg. that stated he was to be the insurer of the kids.  If he didn’t provide it in such-n-such time, they were going to drop coverage on ‘em.  And, perhaps, recoup costs.  Just asking. . . .

Which brings me to the point that as much as we may love our SKs and want what is best for them, sad to say, we also have to be careful what we do for them.  It is not quite as severe as whenever you do anything for them, don’t expect anything in return, but almost.  We all like to think that kids will be kids and when they get older, they’ll appreciate us for what we do for them.  This does commonly occur in initial families.  However, this does not necessarily occur in the least in blended families.  There are just too many disseminating factors that can come into play for any SP to just assume that because they were nice to so-and-so and did this and that for them, that they’ll get any kind of appreciation in return.  Sometimes, as what may be happening here, our own DHs even (accidentally, on purpose, or accidentally on purpose) sabotage our efforts; if anything, by turning a blind eye to the subtle or not-so-subtle manipulation that is going on, whether that be by SKs themselves or BM or other family members.

So, for any SP, the best advice is to do what you may for your SKs, but take care that you don’t pay too high of a price for your efforts.  If you are the type of person who expects thanks in return for instance, and most people do, don’t overdo for them, esp. if your DH isn’t parent enough to teach his kids to show appreciation for what others do for them.  And, getting back to the ins. above, I’m not saying this is going to happen at all, but just think if your carrying ins. on them was illegal and the insurer went after you to recoup their costs!  Just an example of why you have to be very careful what you do for someone else’s children.  Unlike the bioparents, you have no legal leg on stand on whatsoever.

Sounds like you have given a lot without getting much of anything in return, including respect from either DH OR SD (and I blame DH for that one).

I think it's time you sit your DH down and lay it all out for him. That you've worked your arse off to get where you are, that you have shared the spoils of your labor with him and his kids, and in return you only ask for respect and some chores to help you out.

I'm assuming SD isn't working right now, and she's not in school right now, and there is a pandemic so she doesn't have a lot of places she should be going, so doing three choirs that will take less than 4 hours out of her 24 hour day seems pretty manageable. ESPECIALLY since she has a free car.

Your DH needs to understand that, while you care about him and his kids, they're crossing the line into using your niceties, and if it continues, the niceties will stop. And I'd put it squarely on HIS lack of parenting and HIM allowing her to use you.

Ask why it was acceptable for him for SD to receive a multi-thousand dollar gift without so much as a thank you to you.

Ask why he thinks it's acceptable that you pay for their health insurance but not ask them to contribute to chores, especially from the kid who got the multi-thousand dollar gift.

And, if I were you, from this moment forward, I'd no longer spend my time and energy on them. SD asks for cash, tell her to ask her dad. If your name is still on the title to the car, tell your DH to buy it off you for SD or she will have to start asking to BORROW your car. 

You don't get to have responsibility without authority, and she shouldn't get privilege without responsibility. You'll end up upsetting your DH, but his upset will not be because you're wrong. It'll be because you're no longer making him comfortable. Too bad. His kid, his responsibility. Your help is a privilege he has abused. Sucks to be him.

I agree with Rumplestiltskin too. 

My stepdad bought me a car when I was 16. My mom and I moved into his house which was 45 minutes away from my school and mom's job. He and my mother also took the car away if I was super grounded (which only happened once). But if I needed gas money, you bet I did my chores. I really only got paid for mowing the lawn though. And my stepdad didn't care if it was 10pm and I hadn't taken the trash to the curb, I still had to do it. My mom backed him up too. 

I'd have a chat with your DH. Is the car still in your name? That's how my stepdad did it. Even though he "bought" it for me, it was still in his name. 

Nope! I have already decided I will in no way, shape, or form, co-sign for SD for anything. Apartment, car, etc. Her "mom" can help her with that. She's the parent, right? DH has always had mediocre credit score, so I doubt he would even be able to swing it. She can also figure out car insurance. 

Then again, SD is so incredibly lazy, it wouldn't surprise me if she just "didn't feel like" taking driver's ed. *eyeroll* and sponge other people for rides. DH totally acknowledges how lazy like her mother she is.