You certainly write well considering English is your second language. You and your daughter were able to make the right choices, who knows why your son was unable to do the same.
Children grow up and do not always keep their parent's value system. Your daughter obviously does not consider speaking (gossiping) about her brother to be wrong. I would guess that you are not going to change her, so you are going to have to figure out what your reaction to her gossiping is going to be.