Not too sure. Town twinning has been around for quite some time, certainly pre WWII.
In 1980's a new scheme was initiated within the European Union. The twinning was more or less to have links with a European town(s) similar to your own. Probably another idea to get us talking and a good PR opportunity! Quite a bit of funding was put into it. So far as Birkenhead is concerned I do not know what the similarities were in the 1980's, but I'm quite sure they must have grown miles apart by now. We hear little of these 'Twin Towns' these days. In the 1990's and early 2000's there was quite a drive to bring young people from those areas to Birkenhead, again partly funded by European money I heard of a couple of schools making return visits but not very much about anything else. They are more like cultural cross-overs, but what criteria is needed to 'twin', is a mystery. They do seem to have similar topography, which may be part of it.
Gennevilliers is the site of the main river port Port of Gennevilliers of Paris on the Seine river.
Lorient is located on the south coast of Brittany on the Atlantic Ocean. The rivers Blavet and Scorff enter the Atlantic Ocean at Lorient.
Latina is the capital of the Province Latina and about half is coastal area and a tourist destination. Other than growing flowers, fruit and vege I can't find anything other that resembles Birkenhead/Wirral. It may have a Town Hall.
Sister City (Midland, Texas) is different Sister City relationships begin for a variety of reasons. Generally sister city partnerships share similar demographics and town size. Partnerships may arise from business connections, travel, similar industries, diaspora communities, or shared History
There is yet another body of people who make the choices but, needless to say, it is never explained to the local population.
Joyce Howell from Midland had an exhibition here in 2000. I think a council employee went for visit around about then, as they do! The art isn't too inspiring, much better artists here!
2000 –Joyce Howell- Participating Artist – Cultural Exhibit of Art between sister cities of Midland, Texas, and Wirral, England – exhibition at Blackthorn Gallery, Wirral, England.