A Dark-Adapted Eye by Barbara Vine.
Just to get the obvious stuff out of the way first, there isn't actually any such person as Barbara Vine, the name being a pseudonym used by crime author Ruth Rendell; the division seems to be that, broadly speaking, she publishes the more orthodox police procedurals, including the series featuring cuddly old Inspector Wexford, under her own name, and the slightly weirder stuff (what you might call "psychological thrillers" rather than "crime novels") under the Vine name. This early paperback edition (pictured) makes the connection explicit, most of the later editions don't.
A Dark-Adapted Eye was the first of the Barbara Vine novels (published in 1986, by which time Rendell had been publishing novels under her own name for more than twenty years), of which I own (including this one) eight. Certain themes seem to be common to all of them: complex family relationships, mysterious and charismatic male characters (often with a bit of a spicy gay undertone), mysterious goings-on with small children, the central crime (if indeed there is one) being buried away in the distant past and only brought into the light of day by later events. My first encounter with the Vine canon was watching some of the TV adaptation of the second Vine book A Fatal Inversion in the early 1990s; apparently A Dark-Adapted Eye was done for TV as well around the same time, but I don't recall ever seeing it.
The subversion of the standard genre thriller starts in the first line here: we're told that someone is going to die, and soon it becomes clear that they're going to be executed for murder. So the "whodunit" bit is out of the way within the first few pages. Other questions remain to be answered of course, like who she murdered and why.
Faith Severn, the narrator, is contacted by a true-crime writer looking to write a book on her aunt, Vera Hillyard, the executed murderer we were briefly introduced to in the first chapter. This prompts Faith to re-live some of the events of her past, in particular those relating to Vera and her younger sister Eden. With about 15 years between the sisters, in some ways they have more of a mother-daughter relationship, which creates friction with Vera's son Francis. Further complications ensue when the Second World War breaks out, Eden enrols in the Wrens, and Vera has a second son, doubts over whose parentage prompt her husband to divorce her. After the war Eden acquires a wealthy husband, but after a miscarriage (which may or may not have been an ectopic pregnancy) she is advised that she will probably never be able to have children. As Vera has been ill, and in and out of hospital leaving her young son Jamie in Eden's care, a custody battle then ensues, with ultimately tragic results.
The "who" I referred to above (i.e. the identity of the victim) becomes pretty obvious at an early stage, and the "why" isn't too difficult to work out if you've got your wits about you as plenty of hints are dropped, though the official revelation doesn't happen until right at the end. What makes the Vine books so interesting is that they manage to keep you gripped despite dispensing with most of the trappings of conventional crime thriller narrative - in the case of the best and strangest ones like The Chimney-Sweeper's Boy, King Solomon's Carpet and No Night Is Too Long this includes dispensing with there actually being a central "crime" at all, outside of the protagonists' heads anyway. A Dark-Adapted Eye isn't quite as good as those, being a bit more conventionally structured, but it's ruthlessly gripping nonetheless.
Just to complete the TV adaptation round-up, No Night Is Too Long was adapted in 2002, starring Hustle's Marc Warren among others, and Gallowglass, the only one of the early Vines I haven't read, back in 1993, and featuring a very young-looking Michael Sheen.
A Dark-Adapted Eye won the Edgar Award in 1987; my brief list for this one goes: 1965, 1970, 1981, 1984, 1987.