I think the title was “Ghosts” or at least had that word in the title. I read it in the late 70s from my junior high school library.
The story was about a brother & sister who managed to travel back in time to know the children who lived in the same house. (I think they turned out to be related.) Their Uncle was involved, and he explained time travel as some sort of wheel within a wheel (I never grasped the concept). The selfish immature girl who lived in the house couldn’t see them, but could hear them when they spoke. When she proudly modeled a new dress and asked her mother for an opinion, she only heard the voice of one of the children say “I think she looks like a stuffed sausage!” When they accidentally broke a vase, I believe it altered their time, and disappeared.
It was made into a movie at some point, because I found it on TV once, but I believe it had a different name.
I wish I knew more, but it is all I can remember. I’d love to read it again and share it with my children.
Thanks so much for trying to help!