Druella favoured Narcissa out of her three daughters. Whereas she basically ignored Bellatrix and Andromeda, she poured all her negative attention onto Narcissa.
Constantly critical of everything the youngest Black did, Druella continually pick the blonde witch apart. Sit up straighter. Be thinner. Smile sweeter. Poise your posture better. Act more like a lady. Stop showing emotions outwardly.. the list was an endless array of put downs until Narcissa had become too afraid to speak in her mothers presence.
This made Bellatrix shake with anger.
The final straw came when Druella had taken to sending Narcissa owls at every meal time in Hogwarts stating her to 'watch your ever growing waist line' 'men don't like fat women' 'think of your hips' until after months of the constant letters, young Narcissa stopped eating all together.
Near collapse due to weakness from lack of food, an owl arrived at dinner time carrying the same sick messages to the blonde at the Slytherin table.
Bellatrix, then in her sixth year snapped, stood up on her seat upon seeing the approaching tawny owl baring its grim mail, and promptly threw such a strong stunning spell at the bird, she killed it in front of the entire school and teachers table.
Bellatrix was facing being expelled from Hogwarts until Professor McGonagall learnt of why the eldest Black had acted out in such an aggressive manner.
Least to say, Bellatrix was not expelled and Narcissa never received another down putting letter at meal times again.