It might help to consider this section from the Catechism that treats the 8th commandment:
2477 *Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury.278 He becomes guilty:
- of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
- of detraction who, without objectively valid reason, discloses another’s faults and failings to persons who did not know them;279
- of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them. *
When you consider a discussion of someone else’s faults, ask yourself these questions: Is what I am saying true, a guess (charitable? not charitable?), an exaggeration, or a falsehood? Is what I am saying going to unjustly harm someone or preserve someone from harm they do not deserve or might avoid by having this information–will any good come of hashing over this or disclosing this? Is there a more charitable way of achieving the same end without putting this other person’s reputation in danger?
What you’re talking about may be not only morally permissible, but morally necessary. I would give these two cautions: 1) Attack behaviors or behavior patterns instead of persons. 2) When possible, counsel others to decisions that are wise in a general sense rather than warning them away from particular persons.
On that note, it is wise to avoid misplaced pity or trust that present a near occasion of sin to someone else. Likewise, it is OK to counsel someone not to put this family member you are talking about into a near occasion or temptation towards sin by making sin easy for them. A pastor I had taught me, for instance, that it is not insulting to others to keep your purse with you at Mass. It is, rather, the charitable act of keeping people who are tempted to pick-pocket from having access to a near occasion of sin. If everyone keeps their purses close, these people who might have used a church as a place to commit theft will not have an opportunity to sin,
and that is a good thing for them.
IOW, this is something you have to discern as you go along. The example you gave sounds perfectly fine, provided you don’t get into the habit of discussing this relative more than is necessary to achieve the good end you’re talking about.
I would caution, too, that people with these disorders can act in very spiteful ways. It is OK to stay off of their radar, too. Not going on the record as saying things about them specifically can be smart.
Whenever possible, no matter who you are talking about, word what you say in a way that the subject could hear with the least bad fallout for anyone, including both yourself and them. This is generally true, but it is an especially good strategy for dealing with someone who would harm you if they knew you were saying anything less than flattering about them.
