It is forbidden to relate derogatory information about someone even when the speaker mentions that he himself has the very same shortcoming or is guilty of having committed the same act. This is derived from G-d’s rebuke of the prophet Yeshayah for his having referred to himself as “a man of impure lips dwelling among people of impure lips.” (Yeshayahu 6:5).
Loshon hara cannot be spoken even when it is clear that one is not speaking out of malice or with intention to malign. Thus, one cannot relate negative information about his loved ones.
Sefer Chofetz Chaim, 9 Tishrei, page 66