It depends on what era of the Catholic church we're talking about. The medieval church got very expansive about which relations were illegal to marry, including

It depends on what era of the Catholic church we're talking about. The medieval church got very expansive about which relations were illegal to marry, including in-laws. > until the 13th century, the church forbade marriages with consanguinity or affinity (kinship by marriage) to the seventh degree— a rule which covered a very large percentage of marriages. https://www.thoughtco.com/consanguinity-and-medieval-marriages-3529573 https://www.thoughtco.com/consanguinity-and-medieval-marriag... So it would forbid you marrying your brother-in-law's first cousin, for example, even if you have no consanguinity with them. Even not counting affinity, you'd still be forbidden to marry third cousins, which nobody has had problems with for centuries.