Re: Team Sif/Team Foster?
It'll be fine. He was with Jane for a while in the comics too. Hell, Jane and Sif both sacrificed themselves for the other at different points, so it's not even like they hate each other. Maybe Thor will wind up with Sif at the end of his trilogy (if the first movie earns enough to warrant one--fingers crossed).
"This I grant to you: You may tell your grandchildren that you stood for a time against the mighty Thor."
- Marvel Heroes