Ok, increasing one every alternate row will give you an uneven side but you need to read the instruction again:
Inc 1 st
at each end of the next row, and every following alternate row until you have 26sts.
I bolded teh part that is important. You will be increasing 2 sts on the increase rows, one at each end. I have missed instructions like that before so I always double check nowadays!
Libbie