Wrong. Will you be a man and stand corrected?

Muhammad's lineage:

1. Muhammad
2. Abdullah (father)
3. Abd al-Muttalib (grandfather)
4. Hashim ibn Abd Manaf (great-grandfather)
5. Abd Manaf ibn Qusai (great-great-grandfather)

Zaynab's lineage:

1. Zaynab
2. Umayma (mother)
3. Umayma's father (unknown name)
4. Abd Shams ibn Abd Manaf (great-grandfather)
5. Abd Manaf ibn Qusai (great-great-grandfather)

Common Ancestor:

Abd Manaf ibn Qusai (5 generations up from Muhammad and 5 generations up from Zaynab)

So, Muhammad and Zaynab bint Jahsh share a common ancestor 5 generations up.

In terms of cousin relationship:

Muhammad and Zaynab are 4th cousins (5 generations up to the common ancestor, then 4 generations down)

But hey, why don't you look at the mirror? Note that in the grand scheme of things, it really doesn't matter if you marry your first cousin... as long as it's not overly done.

Here's a list of Christian royalty who married their first cousins:

European Monarchies

1. Queen Victoria (UK) and Prince Albert (Germany): first cousins (married in 1840)
2. King Leopold I (Belgium) and Princess Charlotte of Wales (UK): first cousins (married in 1816)
3. King Alfonso XII (Spain) and María de las Mercedes of Orléans (France): first cousins (married in 1878)
4. King Louis IX (France) and Margaret of Provence (France): first cousins (married in 1234)
5. King John (England) and Isabella of Angoulême (France): first cousins (married in 1200)
6. King Manuel I (Portugal) and Maria of Aragon (Spain): first cousins (married in 1500)
7. King Frederick William III (Prussia) and Louise of Mecklenburg-Strelitz (Germany): first cousins (married in 1793)

Other Christian Royalty

1. Prince Ernst August of Hanover (Germany) and Princess Victoria Louise of Prussia (Germany): first cousins (married in 1913)
2. Grand Duke Jean of Luxembourg and Princess Joséphine-Charlotte of Belgium: first cousins (married in 1953)
3. King Haakon VII (Norway) and Princess Maud of Wales (UK): first cousins (married in 1896)

Wrong. Will you be a man and stand corrected?

Muhammad's lineage:

1. Muhammad
2. Abdullah (father)
3. Abd al-Muttalib (grandfather)
4. Hashim ibn Abd Manaf (great-grandfather)
5. Abd Manaf ibn Qusai (great-great-grandfather)

Zaynab's lineage:

1. Zaynab
2. Umayma (mother)
3. Umayma's father (unknown name)
4. Abd Shams ibn Abd Manaf (great-grandfather)
5. Abd Manaf ibn Qusai (great-great-grandfather)

Common Ancestor:

Abd Manaf ibn Qusai (5 generations up from Muhammad and 5 generations up from Zaynab)

So, Muhammad and Zaynab bint Jahsh share a common ancestor 5 generations up.

In terms of cousin relationship:

Muhammad and Zaynab are 4th cousins (5 generations up to the common ancestor, then 4 generations down)

But hey, why don't you look at the mirror? Note that in the grand scheme if things, it really doesn't matter if you marry your first cousin... as long as it's not overly done.

Here's a list of Christian royalty who married their first cousins:

European Monarchies

1. Queen Victoria (UK) and Prince Albert (Germany): first cousins (married in 1840)
2. King Leopold I (Belgium) and Princess Charlotte of Wales (UK): first cousins (married in 1816)
3. King Alfonso XII (Spain) and María de las Mercedes of Orléans (France): first cousins (married in 1878)
4. King Louis IX (France) and Margaret of Provence (France): first cousins (married in 1234)
5. King John (England) and Isabella of Angoulême (France): first cousins (married in 1200)
6. King Manuel I (Portugal) and Maria of Aragon (Spain): first cousins (married in 1500)
7. King Frederick William III (Prussia) and Louise of Mecklenburg-Strelitz (Germany): first cousins (married in 1793)

Other Christian Royalty

1. Prince Ernst August of Hanover (Germany) and Princess Victoria Louise of Prussia (Germany): first cousins (married in 1913)
2. Grand Duke Jean of Luxembourg and Princess Joséphine-Charlotte of Belgium: first cousins (married in 1953)
3. King Haakon VII (Norway) and Princess Maud of Wales (UK): first cousins (married in 1896)