Need opinions on parenting GF's (34F) daughter (11F)

I'm (36M) a father of two. I have my kids part time. My GF has her daughter full time. I live with them.I'm going to apologize now cause I have a lot of info I'm going to throw out there and it will probably not be formatted the best.Until now I have kept my distance on disciplining the GF's daughter. Recently it came out that her daughter has been hitting her cousins. The daughter is generally a very good kid. She does talk back to her mom some and the daughter can definitely push her moms buttons. (sidenote: the father is not in the picture at all due to some very very bad misconduct with the daughter) My GF will get onto the daughter and the daughter will mouth a little more but back down pretty quick.....generally.Then last night, it came out that the daughter as been hitting her cousins. My GF's brother-in-law and brother-in-laws mother knew about this for the last year and said nothing to my GF. My GF and I are both completely shocked at the allegations because we have neither one seen any sort of violence out of this girl. That being said, i do know that one of the cousins is 15 and can be a bully at times. However, that cousin stayed the night with us the day before all this came out and she seemed perfectly fine with the Daughter and they talked and laughed like best friends.Now for the kicker.....My GF's family has this idea that discipline should be physical. I was pretty abused physically and emotionally as a kid so discipline is awkward for me...at best. My two kids (11M, 6F) have never even been spanked. Both are very respectful and good. My little girl starting to show a lot of attitude, but I attribute that to her mom and I separating.The GF's daughter has been through hell. Her mom has been married twice and both were abusive. Her last BF was very controlling. All three were drinkers. I am the polar opposite of her previous relationships. Good job, clean cut, caring, present, and I don't drink. This is my only serious relationship since my divorce (I was married for 15 years and together for 18) so I'm a little bit of a fish out of water. I teeter on wanting to be accepted by the daughter but also, I see the mother could use some help with parenting and for the first time, she expressed that last night. She is at her wits end.My question/s:How do you start the process of handling this situation?Get the daughters side, obviously.......right?!?!How do you discipline an 11 yr old girl that is generally good but seems to have issues hitting other kids?How do I, as the moms BF handle this?Like I would would with my own kids?Like the mother would?Any help would be GREATLY appreciatedTL/DR: GF's daughter is hitting other kids. GF asked me to help with discipline. How? via /r/Parenting https://ift.tt/2KplEh3