I think my brother has a personality disorder
What makes you think he has a disorder?
He's so impulsive, for example he bought a car with his savings but had to sell it soon after because of its high payments.
Oh, that's scary to think about. Even if he does have a personality disorder you shouldn't treat him any differently because that would be rude. Try to get him some help if you think it's that serious though.
I'm going to ignore his problem and let someone else take care of it.
You're his brother and he's your family so it is expected of your to look after him if he has personality disorder.  You should not discriminate him for being different. You should take him to the doctor provide all the help he needs.