Offspring of sexual reproduction receive 50% of there genetic material from each of 2 parents. Because the parents are necessarily (usually) of the same species, they too will share similar genetic composition. So the 50% received from each will be very similar to each other. Only 50% of the *differences* between parents can passed to the offspring from each parent affecting similarity. Some mutation is possible which can affect similarity. Offspring of sexual reproduction are highly similar to their parents.