Question on sexual immorality and a family situation
It is not officially a breach of the 3rd precept, which, per definition, relates to actual sexual activity with specifically defined people (such as another person's husband or wife or children/teenagers still living under the care of their parents). 

However, pornography is of the kind of extreme "dangerous" (ādīnavā) potentially addictive vices found in the 'six causes ruin a man' in the Sigalovada Sutta. 

Also, extreme pornography is particularly discordant with Buddhist views about the aims &amp; needs of women (AN 6:52); it is often 'sexual objectification'; is a 'delusion' about reality; &amp; thus does not abide with metta (wishing good welfare) &amp; compassion (mitigating suffering). 

Since, it was your computer, you have a duty of care (towards yourself) to mention it to your relative because it was a breach of your trust &amp; something that could potentially have imperiled you (such as if it was an illegal kind of pornography that could be traced to your computer or if it was a work computer supplied by an employer). 

If the relative is your child, again, the Sigalovada Sutta states a parent has the duty to restrain their children from evil &amp; encourage them to do good. 

Similarly, as a genuine friend, the Sigalovada Sutta states to protect a friend when he is heedless, to not forsake him in his troubles, to give good counsel, to discourage from doing evil encourage &amp; to do good. 

The Sigalovada Sutta contains most answers to Buddhist personal &amp; social conduct. 

As for pornography, it is a very powerful (alluring) sense object, even if looked at out of curiosity. Since it is freely accessible on the public internet, many people watch it or have viewed it. 

I have noticed more than a few members on a certain Buddhist chatsite that have brought up their problems with &amp; addictions to pornography. 

About sensual pleasures (which includes pornography) the scriptures advise to be cognizant of: (i) the origin (all lifeforms are sexual/reproductive/filial beings); (ii) the allure/attraction; (iii) the drawback/danger; &amp; (iv) the method to escape the danger (which is appropriate healthy use of appropriate sexual relationship or, otherwise, restraint). Some links below. 

http://www.accesstoinsight.org/tipitaka/an/an04/an04.010.than.html
http://www.accesstoinsight.org/tipitaka/mn/mn.054x.than.html 

Pornography can have a powerful allure but I assume many people grow out of it (just as many people get addicted to it).