How do you get rid of brush marks when repainting
Are the brush marks from the previous painting very deep? Sanding might be an option if they are very heavy brush strokes. 

Another possible solution is to use a heavier nap roller. This will help fill in tiny gaps and is meant to provide better coverage for walls that have a textured surface. But keep in min that the heavier the nap you use on a flat wall the more you will create the orange peel effect. I personally don't mind that but just wanted to point that out. 

Be careful that the paint you're sanding isn't old lead-based paint. You don't want to get that in air and breathe that in.

Any way to provide a photo so we can see what you're working with?