SOLVED. Regency Romance from the 70's or 80's possibly a signet. I think "Duchess" may be in the title. Heroine is plain with brown hair and stepsister is a blonde beauty. Hero is a duke. Book is thin like a harlequin romance. Spoilers ahead. [s]

[Amanda]

The duke is planning to court the stepsister but is caught in what appears to be a compromising situation with the heroine and has to marry her. She is either sent to his country estate or goes there voluntarily. The housekeeper resents her and resists her taking control of the household. Near the end either the housekeeper or the duchess gets locked in the estate's ice house. After that, someone fills the housekeeper's room with birds as she is terrified of them. The housekeeper goes to the duke to complain but sees them together and knows it is useless because they are obviously in love.

[☯ DαякєηRнαℓ ❛ ᶜʳᵒᵘᶜʰᶤᶰᵍ ʰᵘᵐᵃᶰ ; ʰᶤᵈᵈᵉᶰ ᵗᶤᵗᵃᶰ ❜]

*following

[Ayshe]

My Dear Duchess (Regency Royal, #6) maybe?

[Amanda]

I think you are right. Thank you so much. I used to own the book and loved it but it got lost with a few other books in a move.