[J.C.]

It definitely helps to have a sister to lean on when it gets tough in the house. However, they haven't really needed this type of support from each other as they have always stayed fairly upbeat anyway. I don't think that simply being sisters is the reason they have kept so cheery. I mean, how would it have been if it was Charley in the house with a twin sister . They would have fought with each other as well as every other hm in sight imo.

I think they just have great attitudes to life generally and they were probably extremely well briefed by someone.

They don't bitch,
They don't go having little chats with each other in secret corners.
They actively avoid getting involved arguments.
They occupy themselves by behaving quite young, which is probably a very smart way of dealing with the frustrations and boredom.

The list goes on and on, so either they have been very well advised, or they are quite smart cookies, and most probably both.