The wicked godmother - M. C Beaton

Lovely but penniless Harriet Metcalf is horrified when she is named in a nobleman's will as guardian of his ghastly and snobbish twin daughters. Is the innocent Harriet wily enough to cope with intricacies of the London Season - and two of its most eligible bachelors, the Marquis of Huntington and Lord Vere? Harriet views them only as suitors for the twins, while the gentlemen see only Harriet's charms. And soon she is falling for one of them...but a cruel betrayal will be her ruin unless the Clarges Street servants can save her honour while she loses her heart!