I don't think Yuki's actually in love with Tae in a romantic way (and am completely prepared to be proven wrong later). She's desperate for a loving Onee-sama and Tae just so happens to be her Onee-sama of choice..
...Mai's flirting still feels so much more deliberate and directed
In spite of current evidence I maintain my position.
Yuki's obsessed and fascinated with Tae...
These are most probably by design. Mai acting the way she does and how she looks are practically the crux of the series. Yuki doesn't have that "defined" place within the overall dynamic, but she slid in after and as a result of her interactions with Tae, and I think now we're in the process of finding her "place" in the series.
Rather than outright in love, I'd say her being infatuated with Tae is the most accurate description, and it doesn't necessarily matter whether it's a romantic connection or a sisterly one, it might even end up being both, with Yuki fulfilling the very loving younger sister trope/slot. I'd say the biggest evidence of that would actually be how competitive against Mai she is in anything that relates to Tae, the happenings of this chapter only serve to cement and reinforce what was already implied. Either case, I think that Yuki views Tae as far more than a regular big-sister figure, I think that much is rather clear.
In any case and about the recent chapters themselves. I'm not too sure what this series wants to do with itself both in general and specifically regarding Yuki. I don't know how I feel about these developments - do I want any payoff, what would be a good payoff in my book here, was having Yuki as a "rival" even necessary? I'm actually rather fond of her as a character, but in truth, I don't think Yuki was originally planned as part of the series, and I do not think she was ever needed. With all that said, what I do know is that I am very entertained and that I still think this series is surprisingly well-written, all things considered. I want to say fuck it, go big or go home, but I do not know that this is what I want.