It might help if you provided some examples for us. Are you thinking of the Middle Ages or the modern period? Within my own area, I can only think of four really nasty fights over inheritance - William of Normandy and Harold Godwinson in the 11th century, Stephen and Matilda in the 12th century, Richard II and Henry Bolingbroke at the tail end of the 14th, and Richard III and Henry Tudor in the 15th. Of these, two saw stronger blood claimants successful (William and Matilda), and a third was a fight between first cousins who both had reasonably strong connections (Richard & Henry Bolingbroke). Henry Tudor is the only example I'm aware of of someone who lacked a strong blood connection seizing the throne during the Middle Ages, and that's because it was an outright rebellion, not a fight over who was the qualified heir.