When you enter many European cities by road you can find billboards announcing their foreign sister cities. The reasons for these agreements can be extremely different; here I have chosen mainly geographic similitudes between Iberian cities and foreign counterparts. This does not mean that these cities have any formal agreement as sister cities to this day; I just think there are interesting landscape similarities.
Seville, Nantes and Houston are historical river ports; they are at a similar distance to their river mouths, in which there are relevant deep-water ports (Cadiz not far away, Saint Nazaire, Galveston), polarizing the territorial system of their regions. The route from Seville to the sea in roman times was probably similar to the present day itinerary from Houston to Galveston, as the present marshes were then a bay, and the shape of the Loire has also changed over time.
In the three cases, along the estuary there is a succession of significant ecological areas and more anthropic activities (docks, industry, industrial crops…). Hydraulic works are relevant (river bed rectification in the Guadalquivir and the Guadaira, channels as that of La Martiniere or the Houston Ship Channel), and a combination of flat land and relevant roads that has led to build long bridges. Despite some hills as those of the Aljarafe in Seville or the timid hills north of the Loire, these are mainly flat lands, nearly ideal for a limitless metropolitan expansion.
In social terms, Seville is not a rich city; if it was to be an American city, it would rather be New Orleans than Houston, despite the presence of high tech industries like aerospace. Nantes is one of the most dynamic cities in France. Both Seville and Nantes are under a million residents (metro area), while Houston is nealy six millions.