Hanging out with you and her friends outside work is way different than hanging out with just you and your brother. She trusts her friends and knows them, and is willing to get to you know you too in the safe and convenient environment that provides. You and your brother though? Too soon. She doesn't know him at all, and she doesn't know you very well yet either. She's probably not going to that movie and would rather not reply then outright reject the offer or think up some excuse not to go.

Right now she sees you as a potential friend. She doesn't know you well enough for trusted friend status or anything beyond. You'll need to build from there in situations where she feels comfortable and safe. Going to a movie alone with a guy you don't know very well and some other dude you've never met is a hard nope.