Yes, and though I stopped reading to fatten up the chapters, it seems that MC isn't as normal or 990% brute force as we thought she was. For example... She seems to be very, very good at computer or hacking or digital shit, considering she was the cause of a lag Gu Yue's company used to have, she also played around, and one of their main servers seem to have been her file trash bin?
In fact, it seems that brute strength isn't all MC is. (SHE'S ACTUALLY HELPS SMART AND REALIZES THINGS NO ONE ELSE DOES DONT LET HER FOOL YOU.) She's just... too lazy.
Still haven't finished the author's other novels, but I doubt the sister is any of those MCs. For one, none of them have the same surname, though there's the possibility of different surname sisters so there's that.
For two, all of the MCs in the novels are pretty... chill, for the lack of a better word, and don't seem to willingly go look for trouble, while from the little bits I read, the Sister sounds headstrong, hilariously aggressive, tough love, and likes to trip up or mess with MC, and it doesn't sound like any of the MCs in the other novels.
For three, the only MCs with a hint of the cross worlds is the MC of My Disciple Died Yet Again and My Big Brother is Seeking Death Again, and even then, The MCs and the Sister don't add up. The MC of My Big Brother, from the few chapters I saw, only has a big brother and isn't an administrator, she's more like working for them (before being duped lmao), and he personality doesn't add up because BigBrother!MC sounds like she is so done with everyone's (*cough*ML*cough*) shit. As for Yu Yao.... Yeah, okay, I heard she ends up as administrator and replaced the Heavenly Dao in that world (haven't finished that one either haha) so the administrator part sums up, but I can't imagine her being the Sister because of her personality.
HOWEVER if there had to be, I think Yu Yao would be the Sister. Administrator part check. Also both novels are set in a cultivation novel. Still I doubt it though. Does Yu Yao have a sister?
The Sister has only appeared for some chapters, so there isn't much info about her. However she's the reason MC transmigrated because she "rented" her to the Devil God, who in return, was the cause of Gu Yue's transmigration (and purposely gave Gu Yue knowledge of the 'plot'), to help save the balance of the realms.
MC seems to be really, really scared of Sister. She doesn't even say her name.
Considering the author's amazing plot devices there's a possibility this novel is connected to the others, but I think the Sister is an original character of the novel.