A Sound of Crying, by Rodie Sudbery. It was first published in the UK in 1968 as House in the Wood, and published with the new title in the US in 1970 by Scholastic (with biscuits changed to cookies; sigh), and it's just right for 9-12 year olds, and I am cross I didn't find it back when I was that age. It is one of those books that teeters between ghost story and time slip, but since the main character does travel to the past to witness the events that transpired (albeit in her dreams), I'm going with time slip.
Polly and her younger siblings did not want their mother to be ill, and did not want to be dumped on their aunt and uncle and cousin Frederick while she got better. Frederick did not want them either; he wanted his peaceful life as a young scientist (poor kids today--no mercury to play with) to be unbroken. But it happens, and if you are going to be dumped on relations, it is nice when they live in an old house at the edge of a wood....
It is not so nice when Polly hears the sound of ghostly crying. And in the night, the dreams come, taking her back to the life of Sarah, a orphaned girl who lived in the house many years ago, who was a virtual slave to her insanely miserly uncle. Polly is a passive observer in the past, horrified by Sarah's unhappiness, and her life in the present is haunted by the story she's witnessing. At last the dreams lead her to the point where Sarah's life changed dramatically, and Polly can finally convince her family that there is physical proof that Sarah's story was real.
It's a lovely mix of the supernatural and the everyday, with plenty of good family dynamics, a lovely setting, and an intriguing mystery in which the past spills over into the present. As the story progresses, Frederick relaxes away from serious scientist who doesn't have room for fun into the good companion he used to be, and although the younger children are only lightly sketched, they serve their narrative functions just fine. Even if you don't like ghost/time travel stories, do try this one if you like 1960s/70s English family stories. I was very excited to learn that there are sequels, although they apparently have no supernatural elements.
Clearly I should have been following my little sister's reading on Goodreads more closely. The reason I got this from the library is that she asked for the fifth book in the series, Warts and All, for Christmas; I'm not quite sure why she hadn't shared these books with me more directly (perhaps she thoughtfully didn't wish to burden my TBR pile). I am even sadder now that the cheap copy of Warts and All I managed to fine was "damaged" in handling and was no longer saleable. I suspect the seller had a change of heart viz the price.
However, if you live in Rhode Island, you can at least read this one, which I am about to return to the library.
Question: though the book is over forty years old, it did not feel especially dated to me (possibly because I am too), and there was no instance in which a cell phone would have made a huge difference to the plot. But do you suppose modern young readers, so used to their electronics, would notice the complete absence of any electronics and feel that they were truly deep in the dark,dark past?