Damn, xkcd AND botany jokes?? I'm in heaven

The nightshades are at least different species. Their vascular system and general physiology is similar enough that your can graft a potato, tomato, pepper, and tobacco plant together for a full meal and after dinner smoke, but they won't really breed. Cabbage and broccoli can and will cross pollinate.

Selection for seeds

It's not brassica oleracea though, it's a different species, brassica rapa. The same species as napa cabbage, brocolli rabe, and bok choy. Rutabaga is actually a hybrid of the two species.

They're all mustard, so technically they do.

Sometimes my sprouts and cabbage will taste hot/peppery/horseradish-y.

My dog

the stem is sweet, I eat kohlrabi like apples (after removing the peel).