The main problem, as Dan alludes to, is that Catherine was the youngest of five children, and therefore was placed behind them and all of their heirs in the succession. Yes, the throne didn't go to a son, but since Catherine's sister Joanna (Joanna the Mad) was alive, she inherited first, and Joanna had numerous children. After Joanna is Marie, Queen of Portugal, and she had a few children too. Then comes Catherine. You'd have to eliminate all of them to let Catherine be the legitimate claimant. Also, the problem with Joanna is her mental instability led Ferdinand of Aragon, who survived his wife and still was ruling Castile (he died in 1516), to authorise himself to take control of Aragon as regent, and he would be hard to wrest control from. By most accounts he was a bit of a power-hungry figure. You'd have to neutralise him, too...