I'm not sure that a fair characterization of her motive, not at least as she tweeted her apology:

"@randizuckerberg I'm just your subscriber and this was top of my newsfeed. Genuinely sorry but it came up in my feed and seemed public."

Screenshot source: http://www.buzzfeed.com/jpmoore/mark-zuckerbergs-sister-comp...

I wasn't speaking to her motive, only the act. Do what you naturally want to do with whatever enters your sphere of attention. It's not our job to run FB's security, so not a second thought should be given to anything that slips through.