A more thorough explanation:

Lyndhurst's Act was a law passed in England in 1835 that made marriages between close relatives illegal and void. The official name of the law was the Marriage Act of 1835, and it was also known as Lord Lyndhurst's Act.

For example, if a brother and sister tried to get married, their marriage would be considered null and void under Lyndhurst's Act. The same would be true for first cousins who wanted to marry.

This law was important because it helped to prevent genetic disorders that can occur when close relatives have children together. By making these marriages illegal, the government was able to protect the health of future generations.