Inbreeding is broadly defined as the mating of individuals that are related. Strictly speaking, however, all animals within a breed are related. So, in a sense, every purebred sheep producer practices some degree of inbreeding. In most cases this relationship is very slight. Therefore, inbreeding is more practically defined as the mating of individuals more closely related than the average of the breed. This practice includes mating brother to sister, sire to daughter and son to dam.
Aaron, Debra K., "Inbreeding in Sheep" (2014). Agriculture and Natural Resources Publications. Paper 155.