Just saying tho: Sis needs to break her mum out of that self-imposed state of disbelief. Smack her with the hard truth and irrefutable evidence that her husband is a lying dolt, the woman she invited into her house is a viper, and the viper's child is an actual Devil's Egg.
Yes, concubinage was legal in Ancient China, but they also provided that among wives, the legal wife should be given the most importance. Dude's been showing his bias since Day One. The First Wife shouldn't be content to remain blind, ffs. At least grow a backbone.
On the other hand, love how Jiuqing is holding out against her half-sister, her dad, and stepmum! Props to sis! Also looking forward to seeing developments between her and ML!