I have two daughters from a previous relationship aged 11 and 7. My eldest daughter is an "easy" and very lovely child, whereas her sister is also an amazing child but far more head strong and can be difficult at times. If she doesn't get her own way she will sometimes have temper tantrums, she might have to be asked numerous times to do something and is clumsy and genuinely doesn't think about the consequences of her actions. She is very good at school with no complaints ever. Family are aware her behaviour can be very difficult too but when she is great she is amazing. I find the best way to deal with her behaviour is to wait to sit her down to talk about it rather than shouting or making her feel "cornered" when people are trying to tell her off because she is a very kind and sensitive child.
My partner and I recently had a baby. Previous to having our daughter he has found it very difficult to tolerate her behaviour. An example that when she was playing on my phone on a long drive to go on holiday (4 hours) he couldn't tolerate listening to what she was watching so escalated it by turning up the radio and a huge argument began. When she wanted to lye on the babies jungle gym he said don't put your feet on it which then only made her want to put her feet on it. She accidently sneezed next to the babies crib he went crazy because she didn't cover her mouth but he went to the extreme. He then continues to rant for an hour about how bad her is. I try and take a fair stance and explain his handling of the situations are wrong and he would be better to sit and calmly explain why she shouldn't do something as getting cross only makes her worse.
I love my daughter more than words can say and if it continues I feel that I will have to end our relationship.