Is marinating raw fish in sweet fortified wine safe?

Without knowing the wine and the sugar content as well as whether you are adding anything else to the marinade, we can't say much. I would hazard a guess that you should be fine (speaking as a microbiologist...), however it will depend on how fresh the fish is, how heavy a contamination you are likely to have in the first place, and the preparation conditions for the flesh.

Alcohol is moderately good at inhibiting some bacterial species, though not some yeasts (these are what produce the alcohol in the wine fermentation). However, studies have found that enteric bacteria, such as those that commonly cause food poisoning are not inhibited fully by the percentages found in wine, indeed - you would need more like 70% (commonly used in microbiology labs for sterilization of surfaces), or even higher % for bacteria found in the ice in drinks served in a bar to completely kill bacteria.

On the other hand, sugar is actually fairly good in its own right as an inhibitory substance, but you also need fairly high concentrations; in the area of 25% (in the upper range for sweet wines) before it inhibits most bacteria. Below this sort of range, many bacteria will metabolize the sugar and grow quite well.

The best factor you have here is the keeping it all in the fridge - cold is pretty good at inhibiting most bacterial species from growing.