Navigating parenting a challenging stepchild

[keepinghome]

I have 2 kids 6(boy) and 9 (girl) and am divorced. I'm marrying a widower who has 2 kids also - 6 (girl) and 7 (boy) - mom died 2.5 years ago. I really struggle so much with his 6 year old. She is so challenging. These are issues I dealt with in my kids when they were 3,4,5. The only person she listens well to is her dad. I'm watching his kids this summer while he's at work and I'm on a break from nursing school and daily there are multiple issues she fights me on, and then fights more about the consequence I've told her would happen and then I end up just sending her to her room because I'm at a loss! For instance the other day:
ME: "A, you have to keep the legos on the blanket. If they are off the blanket you'll have to take a break and come back to try in a little bit."
Later when the legos aren't on the blanket - "A - you need to take a break from legos now"
A: sits there. "I don't want too!!!"
ME: you're choice is go play something else or you can go to your room.
A: continues to sit there and whine.
ME: it looks like you are choosing to go to your room, please go now.
A: continues to sit there and whine
ME: physically pick her up and carry her to her room.

And there was a situation at a museum (many of the articles were out in the open, not in a case) where I had told all the kids if they touched anything they would need to hold my hand. She touched something after explicitly being told not to, so I told her she would need to hold my hand. She refused, whined, and pulled at me when I tried to hold her hand so I just took her to her dad.

Is there something better I could be doing?! Some magic tool I'm forgetting about?! I feel like I have to have 2 consequences ready because the first one is never enough. All the kids are so easy but I have to constantly be getting onto her about something. I'm sure it makes her feel worse and act worse, always being the only one in trouble. I know she was spanked pretty regularly up until the last few months when my fiance quit spanking. Any advice, book recommendations, etc would be appreciated! I've been out of practice, and I feel punitive.

(Also, so grateful GCM is still around )