What's the Name of That Book??? discussion

Okay, so this is a story I read in the beginning of high school so we're looking at around pre-2008ish. I believe it should be a young adult book, since I did read it in the high school library.
The story from what I remember, is a girl who live in this vast lonely mansion with her father, and maybe a sister? She believes that she can see ghosts, but you find out later in the book that her father froze the mansion in time? There isn't a lot I do remember, except one particular part, a woman ( I think she was the fathers lover? or someone he was in love with?) she drowns in a lake, by putting stones on her skirt to weigh her down.
Hope someone can help, have been looking for years xoxo

You are looking for this book:

You got the drowning right, however it wasn't the father's lover. Instead it was another family member's bride.

DαякєηRнαℓ's suggestion of Century by Sarah Singleton matches your description.

Some Goodreads reviewers say, "Mercy and Charity live with their father Trajan in their old dark mansion Century. They live in an eternal winter and they hardly remember their past. But while Charity doesn't really care, Mercy sees mysterious ghosts of their past. She sees a woman under the ice in the lake and meets Claudius, one of the family members she does not remember." | "This is a creepy book about children who are kept in permanent stasis by a grieving father, and who eventually learn the dramatic events that have led to their imprisonment in time." From the book: "She weighted her dress with stones."

Here is the Google Books preview - https://books.google.com/books?id=dfi...