So what chapter was this second lead introduced?
Also, what was su lou's family's reaction when they found out her true capability?
That depends on what you mean by 'second lead'.
Spoiler:
If June is referring to who I think (and I disagree this is actually a second lead) Su Luo will see that character possibly after chapter 500 or so (there is a secondary male character introduced very soon that I think is much more important though.)
As for the Su family's reaction on Su Luo's improvements:
Spoiler:
Disbelief from the father, hatred from the brother/sisters/stepmothers. Then things escalate.