I don't want to wear a suit jacket to my brother's wedding before his fiance dies
Why is wearing a jacket an issue for you?
I only need it for a short period of time they can be an expensive investment.
There are general rules for how to dress at special events and it is good if you follow these rules. It is a way to show your respect to others.
Fuck that, this is my family not yours.
You should respect the dress code because it looks great. If it is too much money you could try a thrift store or rent a jacket for the occasion.