I feel like there are a few misunderstandings here. (Which happens! No worries.) First of all, Alec "ridding himself" of Viola wouldn't free him up to do anything with Isabelle that he isn't already. He's already raping Isabelle, treating them as a romantic partner, parading them around in front of others when possible, heaping responsibility on them...Viola's aware of the sexual abuse from the time Isabelle is fourteen, and she does nothing to put a stop to it. The only ways Alec could bind Isabelle to him even further are impossible not because of Viola but because of the Underwoods' social standing - in other words, he can't marry Isabelle even if Viola is dead because everyone around the family knows Isabelle is his daughter. I really don't see what more he could do with Viola dead that he can't with her alive.

Secondly, I feel the phrasing "rid himself" implies that Alec sees Viola as a burden or otherwise unwanted. That's not the case. Getting married was Alec's idea, and he's very happy in his marriage with Viola - he enjoys the ways she pushes back against him, the games he can play with her, her own capacity for cruelty, the ways she fills gaps in his life and in his nature and complements his existence. While Alec isn't romantically in love with Viola, he doesn't harbor any desire to throw her away in favor of Isabelle. Fundamentally, he wants both.