The Rose Garden by Susanna Kearsley

When Eva's film star sister Katrina dies, she leaves California and returns to Cornwall, where they spent their childhood summers, to scatter Katrina's ashes and in doing so return her to the place where she belongs. But Eva must also confront the ghosts from her own past, as well as those from a time long before her own. For the house where she so often stayed as a child is home not only to her old friends the Halletts, but also to the people who had lived there in the eighteenth century. When Eva finally accepts that she is able to slip between centuries and see and talk to the inhabitants from hundreds of years ago, she soon finds herself falling for Daniel Butler, a man who lived - and died - long before she herself was born. Eva begins to question her place in the present, and in laying her sister to rest, comes to realise that she too must decide where she really belongs, choosing between the life she knows and the past she feels so drawn towards.