I don't think the Southern EU is much worse than Northern EU. Based on percentage, it appears to be higher. But, nepotism is a relatively fixed valued per employer, not percentage based.
What I mean is, an employer's close friends/family is limited. The amount of family members in the same company is likely comparable between Northern and Southern EU. The reason of Southern EU appear to be a lot worse because the jobs opportunities are way less. If Northern EU gets to the same poor economy as Southern EU, you will likely see the difference getting closer.
Nepotism is natural. Like I said, donation, salary raise, hire more workers, are all luxury items that can only be done when there are enough to spare.