Slightly married - Mary Balogh

Colonel Lord Aidan Bedwyn has come to Ringwood Manor to keep his promise to a dying fellow officer.
He has sworn to protect the man's sister no matter what - even when he finds that Miss Eve Morris wants no part of his protection.
Finally, Aidan makes Eve an offer she cannot refuse.
It is intended to be a simple, straightforward business arrangement - but they have reckoned without their own unwilling attraction to each other.
And soon they begin to wonder if perhaps it will someday be possible to be more that just slightly married