5 year old being very hurtful towards her mother/grandmother

Quick backstory:My fiancé has a 5 year old daughter from her previous marriage. They separated when she was 9 months old. She goes back and forth from us (65%) to her father (35%). She spends a lot of time with the grandparents on both sides especially during the covid-19 lockdown.Lately she has been acting out a lot wanting to spend more time with her father and his grandparents. She has been constantly telling my fiancé and her mother she likes the other side of the family more and prefers them as parents/grandparents because she will get whatever she wants from them. My fiancé's mother has been putting in an incredible amount of effort the last month taking care of her almost to the point of spoiling her with love and affection. Her behaviour has been very hurtful and upsetting for my fiancé and her mother.I have a very good relationship to my "stepdaughter" and we have become very close. The way she acts towards her mother and grandmother worries me and they are not sure how to handle her behaviour. We are expecting a baby boy in July which complicates the situation.I know she is only 5 years old and kids can be irrational. The last thing anyone wants is to push her away or make her feel unwelcome but her behaviour towards her mother and grandmother needs to change. I don't want her to manipulate both sides of the family into a competition of who will spoil her more.Any advice on how to handle this situation? via /r/Parenting https://ift.tt/35eVuoD