Most state laws do not expressly prohibit notarizing for a relative. However, Notaries who do so in many instances will violate statutes prohibiting a direct beneficial interest. For example, if a Notary is asked to witness his wife's signature on a loan document for the purchase of a home they will share, he will directly benefit from the transaction and should disqualify himself.
The likelihood of a direct beneficial interest is usually greater with immediate family members -- spouse, mother, father, son, daughter, sister or brother -- than with non-immediate, such as in-laws, cousins, nieces, nephews, aunts and uncles.
In many instances, a Notary will have no beneficial interest in notarizing for a relative and will not be prevented by law from doing so. However, to avoid later questioning of the Notary's impartiality, as well as accusations of undue influence, it is always safest for a signer to find a Notary who is not related.