Thank you very much for asking! As Alec groomed Isabelle for eventual abuse, he also groomed Viola to accept that abuse. It was very much a frog-in-a-pot thing. He knew that Viola's refusal to recognize that Alec groomed and abused Viola herself would keep her from seeing matters clearly, or indeed wanting to see them at all, so he purposefully groomed Isabelle right under her nose to build up Viola's repulsion - not to Alec, who she remained in love with, but to Isabelle. It was always going to be easier for Viola to reject the child she never really wanted than for her to reject the man she loved.

Viola went years "yes and"ing her own perception of what was happening between Alec and Isabelle. Nothing is wrong, but if something was, it would be because Isabelle is an attention whore. Nothing is happening, but if something was, it would be because Isabelle engineered it. She already hated Isabelle so it was easier for her to put the blame there for what she knew deep down was occurring but what she was able to deny until Isabelle, age fourteen and raped by Alec for the first time, came to her and told her what happened. Viola reacted with rage and accused Isabelle of seducing Alec deliberately and then lying about it in order to play the victim. It'll be a fic eventually, and I'll get to lay out Viola's emotional response - the way her own trauma is triggered, the way she goes on the offensive as a form of defense not only of her own psyche but her marriage and her way of life. It's a Lot.

After the fact, when Alec had returned from his business trip and life went on as usual but with this added awareness on Viola's part that Alec was fucking their daughter, Viola's hatred of Isabelle only deepened as her ability to ignore what was going on was put under further strain. So no, she wasn't pleased about it, but she chose again and again to blame Isabelle rather than risk jeopardizing what she actually cared about.