This is a problem inherent in any form of social media. Unfortunately, it's not easily fixed. You need to find a way to strike a balance between rewarding content creation and punishing spam, and the line in thin.
Some of the same relatives are friends on Facebook, and it is much more pleasant experience, because they are nominal "friends", but are in a high privacy group, so we don't have much opportunity to interact.
The problem with websites like Reddit is that they suffer from the so called black swans. A good post can garner karma in the thousands, but a bad one will only have a negative one or two. So, reposting is a great strategy especially when new content is so hard to come by.