Mark D Worthen PsyD

As a C&P examiner, I took letters from spouses seriously. And, if a vet was accompanied by his or her spouse and asked me to speak with him or her, I would always do so as it often helped me to better understand the frequency, intensity, and duration of mental disorder symptoms and associated functional impairment. 
Unfortunately, VA does not require examiners to interview family members, and there are not any specific requirements for examiners about how to regard letters from family members. 
Nonetheless, letters from "lay witnesses" could help, even help a lot, and they're unlikely to hurt a veteran's claim.
Veterans law attorney Chris Attig recommends submitting lay witness statements as a "Declaration", which he explains in a blog post. Note that Mr. Attig calls it a "sworn declaration" whereas other legal websites, and the relevant U.S. Code, call it either an "unsworn declaration" or simply a "declaration". In my experience reading Mr. Attig's blog over the years, he offers well-written, important, accurate information for veterans and their representatives. I don't know Chris Attig personally, but I respect him and value his opinion.
Although they don't mention using a Declaration, the law firm Chisholm, Chisholm, & Kilpatrick has a good article, accompanied by a question & answer video titled, How to Use Lay Evidence for VA Disability Claims.
There's an easy-to-understand, informative article on the Martindale (legal publisher) website that discusses Declarations in general,  i.e, the article is not specific to veterans law: When to Swear and When to Declare: Affidavit or Declaration? by Matthew J. Bakota (21 May 2012).
Finally, here is the statute itself: 28 U.S. Code § 1746 - Unsworn declarations under penalty of per­jury. I'm not an attorney, but it looks pretty straightforward. If it were me I would write exactly what the law specifies at the bottom of a family member's letter, and ask them to date and sign it:
Note this part: "... under penalty of perjury ...." Chris Attig offers important advice in the blog post I mentioned above:
I hope all your exams go well SwiftSig!
~ Mark