The relationship of a woman and her brother-in-law can be seen as incestuous. From Wikipedia:
Incest is sexual activity between family members and close relatives. This may include sexual activity between people in a consangueous relationship (blood relations), or related by affinity, such as members of the same household, step relatives, those related by adoption or marriage, or members of the same clan or lineage.
Incest is a widely spread taboo, and considered so more often than polyamory, for instance. I can understand the point that this is "more taboo" than poly itself.
I see these things so that everyone needs to find out their own limits and boundaries. Of course the law of each state and country puts some lines in place, but still everyone needs to figure out what is right and wrong for themselves - even within the boundaries of the law.
There are many debatable questions, and this kind of a relationship is one of those (is it really incestuous? is it wrong?). I see this as Neverwhere's inner struggle. He needs to define to himself what he actually thinks; if he is willing to continue breaking this particular taboo or not. As I see it, there really is nothing anyone else can do.
Breaking a taboo is hard - I dare to say that every poly person has encountered some of those hardships. Breaking a taboo can work, if you believe strongly that the thing you are doing is right and that the societal approval is not necessary to you.