well since I ain't reading the book I can't 100% tell you ,but I would guess they ment that they keep bugging you and getting into your personal business
This means that siblings interfere in your life as much as a paparazzi do.
Explanation:
So, this analogy is to exemplify a intromission relation, paparazzi and siblings, they interfere when you don't want to, seeking for personal stuffs and criticising everything you do.
