This is an excellent question, and I'm so glad you asked! My best friend and I have been doing research on this very subject.

The Walsingham family charter was written to permit female heirs in the event that the man holding the title has no sons, and historically the family estate has been inherited by a woman before - that's why Alec is Lord Walsingham even though his surname is Underwood, because a Walsingham heiress married an Underwood centuries ago. So when Alec dies, Isabelle gets preference over the many male cousins who would take her place if she wasn't there. She is technically the heir.

However, you're right - Isabelle is not remotely prepared to inherit, and that's by design. Alec has sabotaged her social life, her education, her reputation, her confidence....She's completely without the tools needed to fill his shoes, and Alec has made no efforts to give her legitimate siblings. Alec has made plans for his criminal empire to be inherited by a young American upstart, Ridley (created by my friend Ira), but both his treatment of Isabelle and his plans with Ridley come from the same place: a complete repulsion from and refusal to cope with his own mortality. Alec refuses to accept that he's a mortal man who will one day die, and that the world will continue on without him and that plans should be made for a future where he doesn't exist. He'd just as soon have everything burn down when he goes.

So while technically Alec has plans for both the estate and his criminal empire once he's gone, they're only plans in the flimsiest sense, and are in a way designed to cause destruction once he - the keystone - is removed.