Spinach is the biggest exception i can think of - _Spinacia oleracea_, in the Caryophyllales, a different order to the Brassicales.

I don't think Indian cooking uses a lot of brassicas as greens. Lots of spinach, plus amaranth, also a Caryophyllale, and fenugreek/methi, a Fabale. Mustard leaf is a brassica, though.