My users ask me to make widgets visibility control ( ips widgets topicFeed, imageFeed, etc ). 

I tried to do it in system/content/widget.php, but, apparently, everything is cached there and even the condition (if member_id = = 1) die; does not work. Any idea or is that impossible?

Use \IPS\Widget instead of \IPS\Content\Widget - completely rewrite widget.

Yes, widgets are designed to cache. You can't really do per-user things in the out of the box widgets, but you can create new ones that don't implement caching if needed.