Iku was the one who was betrayed the most this chapter, that sorrow and betrayal on her face can be discerned without any doubt. And to think Mei of all people would be the one to do it.
To be fair, the ones in the background/periphery of the big group shot are the ones who were in maid uniforms way back in the maid squirt gun battle chapter. So I get why they're not double-dipping.
There's a woman with a similar hairstyle on the edge of that first page, but she's taller than Mei, so she's probably not meant to be Mai. Now of course you could argue that it's a prototype and that the details were filled in later, including the whole "younger sister" thing — but I'd suggest that it's instead Mai's grandmother, the former chief maid.