They are both adjectives related to feudalism. But what is the difference between the two in actual usage.
Feudal also means, "Absurdly outdated or old-fashioned" while feudalistic refers exclusively to a feudal system of lords and serfdom
I'm going to try to explain the difference.
Feudal refers to some well-known feudal system from historic times.
Feudalistic would describe something else, which the author feels is similar to an established historic instance of feudalism.
If you feel that a social system amounts to feudalism, then you could say that it is a feudalistic system.