Reddit mentions of The City of Shifting Waters (Valerian & Laureline)

Valerian might be a good start.

Here you go. Took me a little while of digging earlier today.

Edit: It looks like it's out of stock on Amazon, but I found it at Barnes & Noble.

Edit 2: They're also on Comixology for slightly cheaper.

in the 80's the comics became a little more adulte than before so your sister might want to stick to earlier issue.

all the issue are considered for everybody for french standard :from 10 to 99 years old. their is some issue with nudity (like this and this). a lot more with soft violence (it's not really graphic, they sometimes fight bad guys that's it.).

English version of the comics are available on amazon,the first one being this one

Did you want digital or print? Print seems to be here, digital can be found here. Or were you meaning torrents or repos?