What if Thor had another sister? A younger sister he didn't believe had proper control of her powers. But when Loki attacks New York she's the one who comes to help defeat him alongside the Avengers. After New York though, she decides she isn't going back to Asgard ever again.
After the attack on New York, Loki's mind is deteriorating, due to mental manipulation. Thor decides it best to search for help on Midgard. He won't let Loki go completely insane.