If I had to guess, I would say around 5-6 weeks old. And yeah, they all get around fine with each other, if anything I would say she is the dominant of the three and picks on the others. As to the mom, we have tried, oh man, have we tried.
Friday Afternoon we noticed that one of the feral cats that hang out around our home had Kittens, and wishing to not have any more feral cats hanging around we caught them and brought them inside in order to socialize them so as to adopt them out. The three Kittens seemly weren't quite as feral...