Was it legal for a man to marry his deceased brother's widow (a situation frequently discovered when digging into ancestry)? Answer: under civil law in Britain, not until 1921 and this law was passed only after lengthy deliberations. If you want the finer fascinating details read Hansard:
The difficulty stems from moral and religious scruple: in the Old Testament Leviticus xx 21 reads, If a man shall take his brother's wife, it is an impurity ... with the accompanying threat that such a couple would remain childless. We might argue that the statement refers to taking the brother's wife while the brother was still living, another far from unusual finding in family history research. Yet the accepted interpretation of the biblical rule has been that marriage between a widow and her brother-in-law is forbidden.
Marriage between a man and his deceased wife's sister was another prohibition, lifted (as far as civil law in Britain was concerned) in 1907.
The Table of Kindred and Affinity included at the front of prayer books set out precisely which of your relatives you were allowed to marry. Kindred referred to blood relatives. Affinity meant relationship by marriage: the basis for this was that a so-called in-law became kin because the bride and groom would henceforth be regarded as of one flesh.
This explains the above taboos.
Nowadays it seems possible to marry just about anyone other than your parent, sibling or child.