Desperate to repair and reunite his dysfunctional family, an idealistic young man brings them to an abandoned island mansion in an attempt to relive his nostalgic childhood vacations, only to discover it overrun by giant carnivorous slugs. (HORROR-COMEDY)

I have to admit, I love the "...carnivorous slugs..." - it's weird and original. Find a better description for the MC "...idealistic young man..." means very little in a logline. Give him a genuine flaw and preferably relevant profession such as; arrogant family psychologist.

If his goal is to reunite the family, make the inciting incident something that relates directly to that. How about; after his parents divorce or after his estranged father returns home.

I think this has potential, and with a few changes, the logline could read very well.

It's just wordy, repair and reunite, is a different look at the same thing. Be as direct as possible.

Cool. Pretty much ticks most boxes. My only issue is that id hope his family are bunch of aholes, as who else is going to be fodder for these monsterous slugs in da house?
Good luck.