FAIR is a non-profit organization dedicated to providing well-documented answers to criticisms of the doctrine, practice, and history of The Church of Jesus Christ of Latter-day Saints.
Question: Are the Book of Mormon witnesses unreliable because many of them were related?
It is claimed that because many of the witnesses are related, this means they are not to be trusted.
Mark Twain made fun of this very issue:
And when I am far on the road to conviction, and eight men, be they grammatical or otherwise, come forward and tell me that they have seen the plates too; and not only seen those plates but "hefted" them, I am convinced. I could not feel more satisfied and at rest if the entire Whitmer family had testified. 
This is what is known as a "ad hominem" attack on the witnesses' character. The term "ad hominem" is defined, according to Merriam-Webster, as:
- appealing to feelings or prejudices rather than intellect.
- marked by or being an attack on an opponent's character rather than by an answer to the contentions made.
How, exactly, does being related to someone else who is viewing the same thing that you are make one less honest or reliable? This is simply an irrelevant distraction. When you are going to show something sacred to someone, you certainly don't show it to strangers but to those with whom you are familiar and who you can trust. As such, one would not expect anyone but close acquaintances and family to be so trusted. The witnesses, incidentally, had reputations for honesty.
Some have claimed that this rebuttal is a misapplication of the ad-hominem fallacy. It's easy to claim that an ad-hominem fallacy is misapplied by invoking the fallacy fallacy, which means that an argument can still be true even if it contains a logical fallacy. Thus, even if it's an ad hominem attack, it may still be true! But that is not the point of the original argument being made here. The original argument states that the witnesses are unreliable because they are related to each other and their love and bias for Joseph somehow weakens their efficacy. It is ad hominem to claim this and does not address the consistency of the witnesses, even when their feelings for Joseph turned sour at different points of their lives. It does not address the multiplicity of occasions when they went on record to testify, the occasions when they went our of their way to correct their testimony when misrepresented by the public press, the both tangible and revelatory nature of their experience, the witnesses other than the 11 that saw the plates and handled them, and so forth. The argument is bunk.
The witnesses would, of necessity, be those who were close to Joseph. Recall the fact that the witnesses eventually had disaffected members among them because of disagreements with Joseph Smith, yet they never denied their witness. This gives credence to their testimony over time.
Relationships among the Three and Eight Witnesses
Three of the witnesses were related to Joseph Smith:
- Joseph Smith, Sr. [father]
- Hyrum Smith [brother]
- Samuel H. Smith [brother]
Five of the eleven witnesses were sons of Peter Whitmer, Sr., who had provided Joseph and Oliver a place to translate:
- David Whitmer
- Christian Whitmer
- Jacob Whitmer
- Peter Whitmer, Jr.
- John Whitmer
Two of the witnesses married into the Whitmer family:
- Oliver Cowdery would marry Elizabeth Ann Whitmer in 1832.
- Hiram Page married the oldest Whitmer daughter, Catherine, on 10 November 1825.
The following video introduces all witnesses, both formal and informal, to the Book of Mormon, examines several of the hardest-hitting claims against them, and demonstrates the emergent strength of their composite testimonials.