Is someone there?

Hi everyone!
I am here again :) I need to tell you something. I was sure I had found THE FAMILY. Everything was perfect: the place where they live,  open minded HP, a lovely small HK buuuuuuuuuuuuuuuuut nothing is perfect! a difficult big HK!
Ok, let me explain myself:
- They live in NYC very close to the most important places.
- He is musician(HD), they are so friendly and love doing parties at home. So they use to have friends over and the au pair is included in the parties.
- I don't have to drive, that's a good point because because I don't like it very much.
- The schudle is so good: just afternoons Monday to Friday and weekends off.
How I said everything was lovely.

Well, They contacted me. We started to speak over email and we did the first skype. I met all the family and I really felt they were the family I was looking for.
During the first called I also met their current au pair and the HM introduced me to her and she gave me her Facebook adress and telephone number.
The second step was to speak with the current au pair. She extended with them for 9 more months so It is a good signal. Well, when I spoke with her , she started to tell me a lot of bad things like:
- The mum is not really nice.
- the big HK has lots of problems and she is really difficult. She needs more attention and she can be really good but she can also be really bad sometimes.
- The mum doesn't know how to manage and She use to apologize the HK so at the end the girl doesn't learn. This behave won't change never.
- Things that the HK use to do: she say bad words to the au pair, throw things when is angry and she even escapes.

Well that's not all, the HM has been so clear and direct. She said I should know everything before I got there. So, she said this type of things about the HK:
"The HK knows how to upset the current au pair because the current au pair is a very sensitive person. The HK insults her when she's upset about something or doesn't want to do something the au pair tells her to do, the HK might make fun ofthe au pair's accent or tell her she wishes she had a different au pair or other things she knows hurt her feelings. Then things will escalate because the current au pair gets upset and usually everything gets worse before it is resolved.
I think the heart of the matter is that the HK  is a kid that needs a lot of attention and when she doesn't get the positive attention she desires she tries to get negative attention but then she ends up even more unhappy when everyone gets upset with her.  The more you give her positive attention, reinforce good behavior, compliment her accomplishments and talents, offer hugs and take an interest in what she wants to talk about, the less she will act out. The trouble is that it's hard to find time to do this as much as you'd like while also getting homework done, paying attention to the other kid as well, getting her to eat before karate or do whatever task she is trying to avoid. 
She also seems to believe that adults who are serious and authoritarian all the time are "mean" because they don't give her many opportunities to laugh, be silly or have fun. So you have to show her you are in charge, but with a smile and a joke, not a threat and an argument. Hopefully you know what I mean. "

That's just some information. So I am confused, I don't know what to do. I don't know If i will be able to manage some situations that obviously will happen.
I need to think. They want to match with me but I am not sure.

What to do you think? I know don't exist the perfect family but probably it is too much. Don't you think so?


Heeeeeelp meeee!! lol
All the best,
Mire