Might be spoilers though I don't consider it to be so:
I went through the raws, I don't understand chinese but I surmised that nothing happened between big bro and Nan Nan, if anything Xin Yang and Nan Nan grew a lot closer and it seemed like they confirmed their feelings for each other long ago. The big brother seemed to tutor or listen to what's going on in their lives and they all had a good relationship. I think Xin Yang grew more attached and a bit jelly and they also kissed. I think that's what Nan Nan is referring to in the last panel of this chapter.
Might be spoilers though I don't consider it to be so:
I went through the raws, I don't understand chinese but I surmised that nothing happened between big bro and Nan Nan, if anything Xin Yang and Nan Nan grew a lot closer and it seemed like they confirmed their feelings for each other long ago. The big brother seemed to tutor or listen to what's going on in their lives and they all had a good relationship. I think Xin Yang grew more attached and a bit jelly and they also kissed. I think that's what Nan Nan is referring to in the last panel of this chapter.