Dr. Austin Sloper (Ralph Richardson) disapproves of the man his daughter Catherine (Olivia DeHavilland) loves. In the third act of the movie he threatens to alter his will again as he does in the 1st act.
When Catherine agrees that he should alter the inheritance and takes pen in hand to begin dictation of his new will, Dr. Sloper changes his mind. His excuse for not following through on the threat is that, "I don't want to disinherit my only child."
Richardson plays Dr. Sloper's scene with anguish.
Why doesn't he follow through on the threat? Why doesn't he want to disinherit his only child? She defies his authority and wants to marry a man who will only harm her financially, emotionally and perhaps physically. To disinherit would be to make known his opinions, his disapproval of his daughter's decisions. He does not wish to fund her foolishness.
Catherine would still be well-off due to her mother's inheritance, which Dr. Sloper cannot alter. With both her parents' money, Catherine is simply excessively wealthy. She would not become impoverished when her father disinherits her.
So why not go through with the threat? For three reasons.
- Dr. Sloper, believes that "family feeling is very proper." Were he to disinherit his daughter, he would push against his own values of service and protection of the family.
- This is 1840s New York; a woman of Catherine's station must have money or a marriage to live comfortably or to have any power or influence. To disinherit Catherine would be to take away some of that influence.
- Taking away her money makes Dr. Sloper like the man he despises - Morris Townsend (Montgomery Clift).
Morris is Catherine's feckless fiance who presumably would also have dissolved her fortune. He would have dissolved it, not in one fell swoop, but year after year in extravagance as he has done with his own inheritance. Morris loves no one but himself, his own selfish desires.
Dr. Sloper, on the other hand, cares for Catherine in his own way. He's not a likeable person. In his attempts to teach Catherine to be sociable and gracious he is instead abusive. However, to disinherit would be too cruel even for him.
See also:
The Heiress (1949): Her Mother's Presence
The Heiress (1949): The Garden Muse