Where do you see possible offense:

at his not having time for you?
at his brother not including you?
or at his asking you if you're sure you still want to go (even though he won't be able to be with you much)?

I would think about how often you've met that brother, how well you know one another and what you feel about him. If you haven't met his bride, then I don't see how he could include you very easily in the wedding since that's usually the woman's doings; if you were her friend, you'd be a bridesmaid and your boyfriend could easily be an usher but it doesn't really work the other way around?

I'm a little surprised that you weren't invited for the rehersal dinner and able to drive up with him, etc.? I wouldn't be offended at that but I could see being a little disappointed. Are you relatively newly engaged or haven't met/don't know the family very well yet? I wouldn't be offended if you're relatively "new" to their family.

It sounds sweet of him to check though, make sure you really want to go if there's not much chance of your being with him much and being around all those "strangers." You might want to take him up on that, do something yourself that day.