He is under an engagement to marry you in a week's time; and Lady Janet is determined to keep him (as she is determined to keep every one else) in ignorance of the truth.

She will do that without scruple.

But the inbred sense of honor in her is not utterly silenced yet.

She cannot, she dare not, let Horace make you his wife under the false impression that you are Colonel Roseberry's daughter.

You see the situation?
On the one hand, she won't enlighten him.