Complex sentence: Ferdinand's claim to continue ruling Castile was somewhat tenuous after his wife Isabella's death, but no plausible alternatives existed as the theoretical heir, Joanna the Mad, was mentally unfit to succeed her late husband Philip.
Split into simpler sentences: Ferdinand's claim to continue ruling Castile was somewhat flimsy after his wife Isabella's death, but no plausible alternatives existed. The theoretical heir, Joanna the Mad, was mentally unfit to succeed her late husband Philip.