If it works it's called line breeding, if it doesn't it's called inbreeding. Using related stock, especially if you've meticulous records of every sheep and only ever use stock with a clean bill of health all their lives and with excellent teeth, feet and udders, can work to fix characteristics like good feet. If you use older stock you know they're older because they've not been of poor enough quality to warrant culling. If you bring in a new ram you don't know what he may have in his background - poor mothering, becoming broken mouthed at a young age, poor feet, etc.