Isabelle by Sophia Holloway

From the author of Kingscastle and The Season...
Isabelle Wareham has not seen much of the world, caring for her beloved widowed father. At his death, she finds she is no longer her own mistress but under the guardianship of her unscrupulous brother-in-law, Lord Dunsfold, who sees her as a way to improve his own fortunes.
The outlook looks bleak until events throw Isabelle and the impoverished Earl of Idsworth together. However, Dunsfold is determined to force her into a more lucrative match and Isabelle will need to rise above her circumstances to reach her chance of happiness.