As to how Rui Li ended up there at that moment, there is a simple explanation. Ten minutes ago, Rui Li was in the very same position her brother left her when she heard a commotion. That's not to say that Ji Yao and Rui Fei were loud it's just that her hearing was quite good.
Since Xiaoyu had gone to take a bath, Rui Li took it upon herself to go outside and take a look. As soon as she stepped out in the corridor she heard the noise coming from their bedroom. Curious, she pressed her ear against the wall to eavesdrop.
The two happened to be tussling on the bed which made her mind that had just been flooded with scenes from Stormy Love race at light speed. The scene playing in her head from their voices alone was as fluffy and sweet as cotton candy.