She told them she was adopted and had the same biological mother but a different father.
The boy, now 14, who cannot be named for legal reasons, was 12 at the time of the offence and his sister was under the age of nine, Cheltenham Magistrates Court heard on Friday.
One of the deputies questioned what they had told them earlier about being brother and sister and Ellet admitted they were.
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.
She explained that the room had a partition and the children would stay on the other side of the partition when she and her brother had sex.
She admitted to being involved in a sexual relationship with her brother since his release.