Time Windows, by Kathryn Reiss (1991).
Miranda and her parents are excited to move into a big old house that's been sitting empty for ages, and she's pleased to find in the attic a lovely dollhouse that is a replica of the actual house. She looks into the house through its windows, and sees and hears former inhabitants of the real thing going about their lives! Miranda passes hours of each day watching their goings on, and learns that there are two families that keep appearing, one from the 19th century and one from the mid-20th century. The mother in the 19th-century family is beautiful but terribly cruel to her little girl, Dorothy. And (this is the scary part), the cruel-ness of that long ago mother is haunting the house. Miranda watches the mid-20th century mother get infected, changing from a gentle loving mama to a hateful abusive one and, even more horribly, she watches her own mother (who she calls Mither, which I didn't care for) start quoting practically verbatim the words of the first evil mother, and starting to behave just like her too.
Miranda, with the help of the cute boy next door, finds the reason why the infection of maternal mother clings to the house, and things tilt somewhat to horror when she and the boy find out what happened to little Dorothy. It is rather creepy and horrible. Fortunately Miranda is able to use the magic of the dollhouse to set things right, and all the bad stuff with her mother gets overwritten with the good mother stuff that should have been there from the beginning.
So this is an interesting time slip experience; Miranda is almost entirely a passive viewer of the past, until right at the end when she actually sends an object back in time. I found the premise pleasing, especially the historical detective story aspect of it, and the horror aspects of the whole experience, in which time slip is combined with haunting, worked well. Miranda was neither a compelling nor dis-compelling heroine; she wasn't, in and of herself, that interesting, and it struck me as really implausible how the neighbor boy falls hard for her almost at first sight, and how both of them start taking it for granted that he'll be hugging her and taking her to the movies. Most girls, even if they are involved in time slip mysteries of horror, have a few thoughts to spare for the boy that's hugging on them, but Miranda just takes it for granted....
Final though--not bad, definitely creepy, probably won't ever re-read, though I bet I would have if I'd read it when I was 11 or so.
Bonus Time Trave Book ID Question:
A blog reader just sent me the following request for book id help, which I was unable to provide:
"I read the book in question while in middle school...so it would have been
published before 1991. I think the main character was a boy. He is staying
at some sort of old home-a castle? He visits a room in the attic or tower
and from there is able to slip through a wall? door? mirror? back in time-I
think to medieval England. There may be another character, an older
gentleman who is caring for him?