You can pick you friends but you can't pick family. This is all to true for Spike, Twilight can be a thorn in his side somtimes. "Get up it's amost noon." or "You didn't do it right, do it again." Or "Try this new potion out, don't worry this time you wont spontaneously combust this time." or she just wont give doesn't listen to him. It can get him so angry sometimes, but at the end of the day, they're still family. But what will happen when Spike and Twilight find out that a mentally broken dragon is Spike's blood related sister?

It's labeled sex because Spike's sister isn't very used to pony life.