There is a surprising number of species of milkweed, but our own is the largest and most ostentatious. As of January 2018, APG IV places Genus Asclepias in Order Gentainales, Family Apocynaceae, Subfamily Asclepiadoideae. "https://www.britannica.com/plant/Asclepiadoideae: Asclepiadoideae, formerly Asclepiadaceae, the milkweed subfamily of the flowering-plant family Apocynaceae (order Gentianales), including more than 214 genera and about 2,400 species of tropical herbs or shrubby climbers, rarely shrubs or trees. It was formerly treated as its own family (Asclepiadaceae). However, molecular evidence suggests that the group is evolutionarily derived from Apocynaceae, and thus it has been recategorized as a subfamily by the Angiosperm Phylogeny Group III (APG III) botanical classification system." I'll leave it there.