Yeah, the mechanics of vampirism were not fully explained here, but Carmilla seemed to describe her own transformation into a vampire at one point, when she reported her memories of her first ball and it involved something horrible and the sensation of two needles pricking her neck. So it obviously can spread, but then again the peasants whom she feeds on while at Laura's house just seem to die. Either there's actually a whole horde of the undead by the end of the story, but we never hear about the rest of them because peasants, or Le Fanu's vampires can choose who to convert and who to just kill.

As for the mother, yes - that was fascinating! It made me think partly of aristocratic women going to great lengths to stage suitable meetings and thus marriages for their daughters, which in a way is a sort of social vampirism because it is always about capturing wealth, and partly of cuckoos placing their young in other birds' nests. I don't doubt there are bucket-loads of publications about the many symbolic readings which can be drawn out of it.