Half-sister? They're full sisters. Both are the daughters of Peter Parker and Mary Jane Watson.
Annie now has a codename: spoilers:end of spoilersFirst question: Flashbacks, obviously. Second question: Mayday is a Web Warrior. She can go to the RYV universe the same way Gwen can go to 616.
Spiderling. She wanted to be Spider-Girl, but Peter says it's taken, presumably referring to Anya Corazon.