Pregnant mum concerned as she doesn’t want her baby to turn out like partner’s child

What would your advice be in this situation?

A mum has reached out online as she's concerned her soon-to-be born baby will turn out like her partner's child. The woman says that her partner has a daughter from his last relationship, and she can act up and tell lies. She wrote:

"Expecting our first, due in November. DP already has a daughter, 6 - she’s lovely, however still doesn’t know how to say please and thanks even though DP tries to enforce and encourage it. She doesn’t listen, answers back and when she’s told off (even in a very mild normal parent telling child off way) she cries and tells her dad she hates him. She plays her mum and Dad against each other and makes things up about people (me included. I provide nothing but a supportive, kind shoulder to lean on for her, but she still told her mum I smacked her bum - I wouldn’t dare!). I have no idea why she is disruptive, she can be very spiteful at times. I care for her greatly and feel guilty for fearing she will rub off on our DC. "Am I utterly unreasonable for feeling this way? Do kids tend to be entirely different? I feel manners are so important and think children learn a lot from their siblings. "I’ve never had children before so maybe I’m naive to feel this way..."

Many mums replied to the post saying that she might be overthinking things:

"You are full of hormones and overthinking things. Your child will still be a baby will your step daughter grows up a little."

While others put it down to her mother and father's break up:

"She's probably confused and upset about her mum and dad being apart. Maybe it shows in her behaviour?"

While another day:

"She's 6. Her father has left her mother for another woman (however it happened) and is having a baby with the latter. Have you thought of suggesting counselling for her to help her cope?"