YANBU the bill should be split equally UNLESS it is your wish to have something more expensive and you agree to subsidise your brother because of this.
I am one of four and I am more financially secure than my 3 siblings and I constantly have this problem it really riles me when they don't pay there way. At my mums 60th birthday dinner last year everyone, including my stepdad put down £15 for their main course and left me to pay for all the wine, side dishes and tips on top of my own expenses, this in a dinner for 18 people.
Also the following day at the same hotel my mum and sister put their lunch bill on my account. And no-one offered any contribution to the cake or disposable camera either. Or bought me a drink.
It really makes you feel used doesn't it?