Bro poor sister, she looks so scared of psycho touko.
Yeah, she's pretty much caught between a rock and a hard place.
If she leave things as they are, then later down the line she might hear about a serious situation that took place with her brother, and may end up regretting not doing more to have prevent it, with said regret lasting the rest of her life. On the other hand, if she acts (recklessly), then she risks having chaos unfold as well as potentially having her brother cut off all contact with her, while also painting a target on her and her child's backs as we know that Touko would act to eliminate any obstacles that attempts to bar her path to "papa".
All in all, I can only hope nothing bad happens to the sister.