When Zhu-er first meets the adult ZWJ, he has broken his legs and is being pursued by Zhu Jiu Zhen and her family. Zhu-er saves ZWJ without knowing who he is, and ZWJ, having learnt to be cautious with others, tells her his name is Ah-Niu (Cow!
Zhu-er even goes to get even with Zhu Jiu Zhen over 'Ah-Niu' but she ends up getting teased about her bad looks. In her defense, ZWJ swears that he is willing to and will marry Zhu-er.
After she 'dies' on the remote island, ZWJ tells his Godfather and ZZR that he has long considered Zhu-er his 'wife' in name though they never had a ceremony. In fact Zhu-er told 'Ah Niu' before she 'died' that she could never marry him since she was in love with ZWJ whom she met when they were both children!
ZWJ wrote "Beloved Wife of ZWJ" underneather her name on her grave marker.
Hence she is the 'acknowledged' wife of ZWJ. The article that I was quoting mentions this fact, and in light of how much attention the actress for Zhu-er is getting, I'm speculating that they will focus more on this relationship than prior versions.
Hope this helps!