... I dunno how to feel about this, i love yuri as much as the next guy but the sister ntr-ing her brother still leaves a bad taste in my mouth (I know she's a yandere and her mind is twisted like that but still as a reader i have more negative emotions than happy ones reading this).
This situation is an odd one for sure yeah. The sister seems to WANT the GF to have sex with her brother. But she for some reason toys with his GF herself.
Theory 1:
As a previous of comment of mine was saying above, she may be acting in a role similar to how horse breeders use another different horse to "entice" the intended breeding pair to mate.
It is possible the sis is doing a similar scheme, with her being the "enticer" to get the GF to have sex with her brother.
Theory 2:
That or she has some weird brocon fetish, and she wants to use the GF as a weird, twisted way to have sex with her brother by proxy. I absolutely would not be surprised if when the GF and brother have sex, the sister immediately tries to have sex with the girl to get an "indirect sex" with her brother.