Viscount in Love: A Novel - Eloisa James

Two eccentric orphans bring together a grumpy viscount and the free-spirited heroine who steals his heart in the first novel in Eloisa James's new Accidental Brides series, in which haughty aristats find themselves married to the wrong women.
He wants a nanny, not a bride.
Suddenly guardian to twins, Viscount Dominic Kelbourne is luckily betrothed to a suitable lady—until she elopes. With no time to woo, Dominic decides to marry his fiancée's unconventional sister. Torie isn't perfect, but their kisses are so passionate that society thinks he's actually chosen her.
She wants to marry for love.
Torie has never been able to make sense of words on a page, so she has turned her talents to art. She longs for a man who values her as she is. but marries for the sake of the twins. She doubts Dominic is capable of love, let alone respect, but as their heated debates turn into something more, Torie begins to imagine a life as a wife, not a nanny.
But when the arrogant viscount finds that his viscountess has stolen his heart, he'll have to give all he has to win her love.