The Federal Child Pornography law only applies when a person is in possession
of the pornographic image. It is questionable had the relative succeeded whether the captured image would actually qualify as pornographic, as the statute requires it to be "sexually explicit".
The South Carolina laws are the same:
Engaging a Child for Sexual Performance. S.C. Code § 16-3-810.
(a) It is unlawful for any person to employ, authorize, or induce a child younger than eighteen years of age to engage in a sexual performance. It is unlawful for a parent or legal guardian or custodian of a child younger than eighteen years of age to consent to the participation by the child in a sexual performance.
(b) Any person violating the provisions of subsection (a) of this section is guilty of criminal sexual conduct of the second degree and upon conviction shall be punished as provided in § 16-3-653.
Possession of child pornography.
South Carolina law provides that the mere possession of any undeveloped film, photographic negative, photograph, motion picture, videotape, or other recording of a child engaged in sexually explicit conduct is a criminal act that may charged as a Class I felony. In order to be found guilty of possession of child pornography, the alleged defendant must know that he or she possesses the material, know the character and content of the sexually explicit conduct in the material, and know or reasonably should know that the child engaged in sexually explicit conduct is under 18 years of age.
Exhibition of child pornography.
If a person exhibits or plays a recording of a child engaged in sexually explicit conduct when the person knows that he or she has exhibited or played the recording, and knows or reasonably should have known the nature of the content beforehand, and knows or reasonably should have known that the child engaged in the conduct was under the age of 18 years, they may be charged with a Class I felony.
The relative could be potentially criminally liable for an attempt to violate the above-referenced state laws. However, the fact that the image was not captured and that it would be debatable that the image would be in fact "sexually explicit" removes the likelihood for any actual prosecution.
To sum up, no crime has been committed under a conservative reading of the federal and state laws. Under the most expansive reading of these laws, only a potential attempt crime of the State law could be said to have been committed. Again, this would be a very expansive, over-reaching take on this law.
For sure you and your wife, as the home owners are not guilty of any wrongdoing under these statutes.
I hope this helps and am glad you prevented something bad from happening.