Marvel's being a little liberal with the "Hell Hath No Fury"... quote, but the implication is clear. Thor and Loki will find out they have an apparent long-lost sister (or at least someone posing as one) in July in an Original Sin tie-in.
There wasn't a "classified" July solicitation for Thor: God of Thunder, but come to think of it, there was no solicitation for Loki: Agent of Asgard. Smart money may be to look there whenever Marvel updates the July solicits, presumably after C2E2.
So, who do you think the sister could be? A new character, an established one?