Thanksgiving...and why are men still cavemen...?

My husbands family lives pretty far away, too far to drive so we are flying there for Thanksgiving. His brother lives in the same state as us but 3 hours away. A plane ticket is going to cost us about $300 each, for him myself and our 1 yr old. I just found out this morning that he wants to buy his brothers ticket and his brothers fiance. We had a huge argument about how he makes these decisions without even consulting me first. How do I make him see that we really shouldnt be buying these tickets for them? Also, we don't make more money than them, we have a child, a mortgage a car payment etc. they have nothing but a car payment. He thinks he should do it because hes the older brother. Why do men always act like there is something to prove?