Give up any hope of having a real or a nice relationship with you.  She resents your existence.  Go for a polite, civil one.  That's the best you can hope for.

Your SD is not your friend. Be civil and expect civil, thats all you get. Dont bend over backwards to please her, nor do you overextend yourself for her children. Civil.

You are not alone. We've been married 24 years. BM and DH were divorced and she died (before we met) when when my SD  was about the same age as yours.

My SD and her husband were very clear when the kids were born (16 years ago) that I am not a grandmother. I decided they get what they want. I may help DH pick out gifts, but I don't buy or pay. Gifts and cards come from him only. 

We are civil and polite. Works for me. To the kids, I am the person who lives with their grandfather.

Just ignore her.  If you point out the backhanded, passive  aggressive comments,  your DH will never see it.  He'll just blame you for interpreting it incorrectly,  he'll accuse you of not liking his daughter,  etc.  Just smile and say "well bless your heart" and give her a big smile.   Lol.  

Also, stop doing anything for her and her kids (if you are).  A saying we have here is "not my circus, not my monkeys".

That's how long I tolerated poor treatment at the hands of DHs ADULT kids. Now, I'm am completely disengaged. DH could not or would not stand up to the crap behavior of his kids so I removed myself from it all. He sees them away from our marital home (a boundary set in counseling) and I don't get involved with his kids. He attends birthday parties or other events alone. While they always invite me (trust me, only to make themselves look good), I do not accept as I know how I will be treated once at said event.

Search this site for disengagment techniques. As someone said, STOP doing any and everything you do for this SD. That would include the grandkids you aren't allowed to be a "grand" to. (I know exactly how you feel on this point also). When she is around, make yourself scarce. Let your DH handle all the conversation while you sit there quiet and smiling. Don't ask questions. Don't get involved in her life. As my pastor once told me, treat them like you would the cashier at the store. You make small talk but nothing deep. 

Best to you. It's not an easy journey but learning to disegage and stop getting involved with your SD will help you immensely.

Don't be around her that much. Limit the time you spend with her as much as you can. 

I'd be ready to divorce a dude if he has the audacity to put some adult kids in my role. 
 

Tolerate that crap for 18 years because of the legal obligation but once the court of law says the legal obligation is over, so is the toleration of men who pretend they cannot understand how to be real men/husbands. 
 

As far as being a grandmother, sorry but I highly doubt anything special about those grandkids. They're just like any other grandkids and in fact you could chose to be a godmother/godgrandmother to someone else kids who you're actually close to. 
 

Forget that passive aggressive leech you don't need her for validation, for grandkids, for anything.....
 

****My stance is a little more extreme and I've never married a divorced dude with unruly skids but I am in process of breaking off an engagement to a divorced dude with untrained no boundary mini-spouses. 

any other toxic adult.

Make no consideration of her losing her mother. Even if she had lost mommy when she was a child, her toxic adult crap should not be tolerated. That she was ostensibly an adult when mommy croaked, she does not even get the orphan kid excuse.

meh

As for her father,  did his balls get lost when his XW passed?  Why is he tolerating his shit kidult spawn to disrespect his mate?

Is what you get this is real life. SD doesn't care about you. Your her fathers SO,  nothing more. For entertainment she plays. Give  Roxy a crum and watch her dance.   Disengage from SD. Make it like she doesn't exist.