I’m getting confused with how the La Rochelle characters are related to one another; is there a family tree anywhere on the board to explain it all?

There's some explanation of the connections in this topic

viewtopic.php?f=39&t=13687

Thanks, that makes things a lot clearer.

Is Nan Blakeney somehow adopted into the Temple family?

Only unofficially. Janie describes her to Jo as an adopted sister Jo who then mentions her adopted sister Robin. But neither Janie nor Jo had living parents at the time who could have adopted a sister for either of them. Nan and Robin are both orphans who make their homes with close friends who are a few years older, and think of them as younger sisters.