I could very well be wrong but I think what it is with the young Lady is that she does like Mia, she just also wants Chloé/wants Chloé bc she doesn't have her or bc she's her moms maid
Rereading through the chapters and it seems like young lady's focus is solely on Chloe. I can't say if that's because Mia is already hers, as shown in the first chapter, or due to anything else. She's also upset with Chloe being very clearly attached/attracted/yes to her mother, whom she's already upset with. That's why she mentions something along the lines of her mother stealing everything she wants. Actually, looking at chapter 1 again, it does seem like she's upset about her mother and Chloe's closeness.
In any case, for some reason I find this series incredibly fucking hot. I was pretty firm on my pairings before, and I still expect, hope, and almost demand more Mother/Chloe shit, but seeing the two sisters in this chapter just made me fucking cast any reason aside. Also I didn't necessarily expect either of the involved parties to be as proactive as they are, or more simply - shit's escalating hella fast, and I'm definitely all for it.