At first sight, Yosuga no Sora seems like an innocent anime like most others based off eroges... A world where everyone is gone except a boy and a whole bunch of girls. And a very moody feel of love and romance etc. The first part was the standard introduce all the girls thing:
The rich girl
The serious one
And the incredibly shy class monitor.
And also, his very bratty and very spoiled sister. Yes. There are incestial undertones here
The funny thing is, at the end of the episode, the entire theme of the show changes from serious and moody to completely whacked and outrageous fun.
With violin string plucking in persian scale.
And there was even an NSFW scene. Amazing that this happens. Usually the producers would perform the steam cloud or glare operation to make people buy the DVDs.