I'm so glad you're enjoying! Thank you very much for reading. Isabelle leaves Alec when she's twenty-two, after Viola overhears her letting slip to her lover Maggie Beth that Alec killed Viola's lover Andre. To get revenge, Viola convinces Isabelle that this indiscretion and lack of loyalty would spell death for her if Alec found out - and Alec always finds out. She helps Isabelle to escape in order to rob Alec of his love the way he robbed her of hers.

So Isabelle breaks contact not because she realizes what Alec's doing to her is wrong, or because she loves him any less, but because she's terrified of losing both his love and her life. Hope that answers your question!