Review Detail of PurpleWinterSept in Music Is Love!?

I like the plot and everything but after Ana gives birth the story starts fetting confusing. the story starts focusing on the four sisters which is where I got confused like how does Ana and Lin Shen know all 4 sisters and the Tiger brothers? they are never mentioned before accept maybe Veta and there is too many gaps to fill in for some of its character. It's a good story but it is confusing.