So, the difference between 兄{あに} and お兄{にい}さん is merely the level of formality. Based only on the rules of 敬語{けいご} you say 兄 or お兄さん. note that the reading of 兄 changes by adding お .
I am almost certain that 敬語 has nothing to do with deciding whether to say 腹{はら} or お腹{なか}. Even though the superficial difference is 美化語, the difference is not about 敬語. So, what is the difference in meaning and usage? Would a doctor refer to my abdomen as 腹 or お腹? Is my へそ in the middle of my 腹 or お腹?