You have described them down to a T. Ni’s mum is so loving and it all comes so naturally to her With Wendy you get a backhanded compliment and that cackleWith regards to Ni's mum, I really dont think anything happened. I just think both the women are polar opposites. Wendy shows her kids love but she is not affectionate, she's very cold sometimes to them. Clearly believes in tough love and her way is the right way aka tries to lead her kids to a life she wants for them whereas Claudia is completely the opposite. She's a smart and successful who is openly verbally and physically affectionate with everyone she comes across even Casey. She is so supportive of her kids whatever they decide to do even when she might not agree with it.
I think theres resentment on Wendy's part sometimes especially in the early days as Harlow was definitely a different child when she was around Ni's family. But I think theres a slight language barrier too.