Libra embraces neutrality. The scales represent fairness and justice, hence they could not be ideal secret keepers because they don't condone gossip.
But it's their fairness that makes them trustworthy. They may not like gossip morally, but they'll listen to you vent if you have anything to say.
Capricorns adore gossip, unlike Libras. Some appreciate it because it keeps them in the loop, allows them to establish a red-string theory board, and lets them see drama without being affected.
Regardless, they'll gladly listen to your secrets and happenings.
Pisces hold secrets because of respect and because they cherish your friendship if you come close enough to share secrets.
Pisces will avoid anything that might endanger your connection since they don't want to lose you or hurt you, especially by themselves.