In

> Winnie saw the truth at once, knowing as she did the character of her,
> whom, if she had ever looked upon as a mother, must from this moment
> forfeit every claim upon her feelings, unless it were that of utter
> contempt.

the parenthetical and other additions to the matrix sentence complicate analysis.

Stripping to an easier but comparable (grammatically at the salient point) stripped example:

> Winnie met the woman who must acknowledge that she had no right to
> expect any sympathy from Winnie.

The parenthetical in the original does not control the grammar of the matrix sentence, so nominative 'who' is the correct pronoun.