"whom...must...": is this real sentence grammatical?

I encountered this curious sentence on page 234 of the 1859
novel Natalie; or, A Gem Among the Sea-Weeds, by Emma V. Hallet
writing under the pseudonym “Ferna Vale”, marked here in bold:

In a few words he informed her of what he had learned from Mrs.
Santon the day previous, but what was his astonishment to find
her totally ignorant of the circumstances, not hesitating to
declare the whole a base falsehood.

“I had not a doubt of the falsity of the report,” said
Delwood; “but what can have given rise to such a statement?
Surely, your mother would not wish to injure my feelings, by
repeating what may have originated, without foundation, among
the servants, and she could not have herself credited!”

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.

“Mr. Delwood,” said she, raising herself to her full
height, her slender fingers clenched together, every nerve
’roused to action, — “if you would not insult me, never
again call the woman who has had the heart to cast such a slur
upon the character of her whom we know is innocent, my
mother! It is not to injure your feelings that she has
invented such a vile scheme, but it is by injuring Natalie’s
character in your eyes, she may banish from her heart all
future happiness. Nay, do not start at such a strange
declaration from my lips; you are the only person, out of my
father’s household, who has a suspicion that our happiness is
not what it once was; but since it has come to this, I will, at
the risk of disclosing to the world what it were wisdom to
conceal, establish the innocent; and rest assure that what I
say is true, — this has originated not amonth the servants,
for there is not but would kneel and kiss the very ground upon
which are dear Sea-flower treads.”

(The context is that Winnie’s new stepmother has proven to be an
ill-natured woman.)

The whom seems to be correct (accusative case, I think: she had ever
looked upon HER), but then the “must from this moment…” feels
disjointed, not properly connected to the first part of the sentence.

Is this use of whom in fact correct? How would one analyse the
complete sentence in question?