I found this article and looked up the book on Amazon:
http://news.bbc.co.uk/1/hi/entertainment/387204.stm
Having done more research into the way TB spreads: droplet infection, like a cold; it's quite possible that one of the sisters (Emily died first, after Bramwell who it's thought died from alcoholism and drug abuse) developed TB then infected a sister who was nursing her. Charlotte died several years after the others, apparently from weakness and extreme morning sickness while pregnant with Nicholl's child.
The UK doesn't have mosquitoes, it's too cold here. But the water supply did run through the graveyard, I think I saw that on here:
https://www.youtube.com/watch?v=QyTeDZZBphI
(If you don't want to watch the whole thing, start at around 13 mins.)
After Charlotte died, Nicholls stayed with Patrick, the father, till his death then went back to Ireland to marry his sweetheart. Nicholls lived to be 88.
Could be entirely innocent, but far too many interesting coincidences not to set the imagination working. I'll probably buy the Kindle version of Tully's book despite the mixed reviews.
(No, I'm in no way affiliated, I've just always found this family particular interesting and gossip-worthy, even today.)