Not all congenital cataracts are hereditary. Some congenital cataracts are caused by some other factors. However, many congenital cataracts are hereditary. Even if there is no history in the family, they can be caused by a hereditable mutation in the infant. Thus the descendant of the infant may get congenital cataracts.
There is study showing that congenital cataracts are rare and occur in developed countries with a frequency of 30 cases among 100,000 births with a further 10 cases being diagnosed during childhood. But if there is family history of congenital cataracts, the chance can be higher.