Ya Brain tan is one of the toughest leathers to cut, at least cut straight, no idea on sharping scissors but I will say I have several pairs that the guys around the shop have used to cut sand paper and they won't cut nothing anymore. Miss Joanie just a couple of weeks ago bought me a nice , pretty pricey pair and they cut brain tan like butter, they are in a sheath and I plan on keeping them there and out of sight of everyone but me. They do have a slight curve in the middle when you look down on them, I think that is to keep good pressure on the front 3rd of the blade. I also have one of the rotary cutters Aaron mentioned and it work great also.
Pappy