The court was told the boy had typed in search terms on the internet in order to find incest porn.
Mr Fenny said "Cases of this nature will increasingly come before the court because of the access young people now have to hard core pornography."
The boy pleaded guilty to six charges of a raping a girl under the age of 13 during the summer of 2015.
District Judge Joti Bopa-Rai sentenced him to a 12-month referral order and imposed a Sexual Harm Prevention Order for five years.
The conditions of the SHPO mean the boy, who is now living in a supervised treatment unit, cannot associate with anybody under the age of 16 and cannot contact his sister.
He is allowed to access the internet but is not allowed to delete the search history because police will regularly inspect all his electronic devices.
Judge Bopa-Rai warned him: "If you bump into anyone under the age of 16 then you have to walk away - you cannot associate with them.
"When you come out of the treatment unit at 16 you will still have police monitoring you until the age of 19."
Because the boy has no previous convictions and is under the age of 15, he would have to be a persistent offender to receive a custodial sentence, Judge Bopa-Rai said.
His father asked the judge if his son would be able to return home once he left the treatment unit, to which she replied: "I cannot answer that.
"A lot will depend on where he is at that stage."
Defence solicitor Gareth James described the offence as "experimentation of a sort, albeit completely inappropriate".
He added: "He is now 14. At the time he was only 12 and that is perhaps significant because it is always the case with youths that they mature at different rates and this has been a lesson for him in terms of coming to court and being interviewed.
"There is a psychological report that shows he shows a level of maturity for his age.
"It is part of that maturity that gives rise to this offending. It has always been the case that teenage boys develop attraction to opposite members of sex, or of the same sex, of course.
"When I was a young man the internet didn't exist but unfortunately it does now.
"Therefore, he does seem to have been exposed to material that was inappropriate for somebody his age and that has played a part in his offending because he has tried to enact what he saw with his sister.
"This is experimentation of a sort albeit completely inappropriate."