Why did Miss Murdstone scream when she saw the baby in David's arm in David Copperfield?

The moment you're referring to occurs in chapter 8 of the novel, when David is home from school for the holidays. David throws Miss Murdstone into a state of "violent consternation" when he "took it [the baby] very carefully in [his] arms." David describes the scene:

She [Miss Murdstone] was limp with horror; but stiffened herself to make a dart at me, and take it [the baby] out of my arms. Then, she turned faint; and was so very ill that they were obliged to give her cherry brandy. I was solemnly interdicted by her, on her recovery, from touching my brother any more.

Later in the chapter, when David's mother remarks that David's eyes and the baby's are alike, Miss Murdstone flies into a similarly upset state, saying,

Who else could compare my brother’s baby with your boy? They are not at all alike. They are exactly unlike. They are utterly dissimilar in all respects. I hope they will ever remain so.

Miss Murdstone resents the implication that the baby, who is her nephew, bears any resemblance to David, who is not biologically related to her and who is, in fact, a person she loathes, as we see through her cruel, inhumane treatment of him. Her disdain for both David and Mrs. Copperfield does not carry over to the baby because the baby is her brother's and shares her family lineage. As such, she wants to "protect" the baby from what she sees as the "stains" on the household: the presence of Mrs. Copperfield and David. When she sees David holding the baby, she reacts so dramatically that you'd think David had a contagious illness that was fatal to anyone who came in contact with him. What she's really doing is manipulating David's own relationship with his mother, testing family ties, and making David feel as though he's an "outsider" in his own home. Her compulsive need for control and to exert that control through displays of power is highlighted in this episode, when David is made to feel ashamed for holding his half-brother.