For me, foods in the same genus have always cross-reacted. There's a plant foods relationships list with about every plant you might eat or someone has ever dreamt of eating.

Brostoff and Gamlin's "Food Allergies and food intolerance" has a good section on cross-reactions at the end. My experience has borne out a lot of what they say:

Someone who reacts to soy is likely to react to all legumes. True for me, I've gotten sick from every legume I've tried.

Same thing for fish. If you get sick from one fish you probably should avoid all of them. Maybe cartilaginous fishes like shark would be OK, since they aren't closely related to bony fishes, the kind of fish people usually eat.

Same thing for shellfish and crustaceans (scallops, lobster, and so on). If you can't eat scallops (a mollusk) you probably can't eat lobster (a crustacean).

Similarly for tree nuts, they say. Even though tree nuts come from many different plant families, they're likely to cross-react. I've only tested one nut - a tsp. of hazelnut oil - so I don't know about this.

Same for poultry. If you can't eat chicken you probably can't eat duck either.

If you react to one kind of bird egg, you'll probably react to all other kinds of eggs.

They say cow's milk cross-reacts with goat's milk but less often with sheep's milk. Probably it won't cross-react with milk from animals that aren't in the cattle family, like horses.

There's more stuff in Brostoff and Gamlin's book about cross-reactions between pollens and foods, latex and foods, etc.