Instance of "happy" sibling incest in Reginald Hill? Which book?

Okay, this is how I remember it--in one of the Dalziel and Pascoe books, it involved two minor characters, brother and sister, who grew up apart. The brother (possibly with some mental disability? autism perhaps?) moves to a place (possibly after being in juvenile detention?), barely-known sister comes to visit him (possibly helps him get a place?) annnnd, somehow or other they end up sleeping together... I think a longer relationship ensues but they fall off the radar of the main plot.

I remember it as being dealt with perfectly casually, even jovially, and, IIRC, the characters didn't "pay" for this, at least not within the scope of their presence in the book.

Anyone know which title this was? I might add I'm about 95% sure it WAS a book by Reginald Hill, partly because of that "jovial" tone I seem to associate with the anecdote, but also because he's basically the only only-recently-dead crime writer in English I've read in the last five years.

I sure don`t remember it sweetie. Did you ever track down this book?

No, I haven't. Later I remembered I had read some story collections of Hill's too, it might have been in one of those and not the main novel sequence. But, since everything I still have by him is in storage, checking up is very inconvenient. Either way, not a biggie... :)