Demographic and social changes over the last century have altered the types and proportions of the different collateral relatives. Birth control has resulted in smaller families with fewer brothers, sisters, uncles, aunts, nephews, nieces and cousins. The increase in divorce and remarriage or living with several partners has raised the numbers of half sibs, half uncles, half aunts, half nephews, half nieces and half cousins. It is quite common now for a person to have two half sibs who are not related to each other,
E and F are half sibs and F and G are half sibs, but E and G are unrelated. Thus, it is possible for F's half brother (E) to marry F's half sister (G) without infringing any forbidden marriage laws. Also, the tendency for more people to live together without getting married, means that many relationships are not being officially recorded for future reference.
With an increase in sperm and egg donation and surrogate parenthood, it is important that records are kept for future checking to prevent close relatives unknowingly having children together. Records are necessary because of a phenomenon known as 'assortative mating'. It is well documented that the phenotypic correlation between husband and wife for certain characters such as height, intelligence and even facial characteristics is as high as between full sibs (see Libby and Lerner, 1976). This means that people unconsciously choose a partner with certain attributes like themselves (or their parents). Therefore, if close relatives are not aware of their relationship, there is a real danger that they could be attracted to one another later in life with harmful consequences.
The taboo which once prevented marriage between certain in-laws and step relatives has now practically disappeared. This is a sensible development which is long overdue. (See here).
Finally, the policy in China of limiting family size to one child will eventually eliminate all collateral relatives; although the birth of twins in a first pregnancy can not be forcibly prevented.