i just remembered i created this topic after got a notification someone replied, haha.
actually i got the answer long time ago.
so the answer for my own question is ... Luo Shuiqing
like @HaruPA said, she is an original character of the novel, not make an appearance in drama.
perhaps the two met after the event of Dashing Youth.
She passed away while giving birth to Sikong Qianluo.
i think the drama producer change this character to Feng Qiuyu is because to add extra "Romance" element to drama version, because Luo Shuiqing had not met Sikong Changfeng yet in dashing youth timeline?.
