All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If this woman is religious and has a good moral conduct, then your brother has to endeavour to convince his parents to let him marry her. Being a divorcee is not a religious obstacle to marrying her, and his parents should accept his proposal to marry her. Since they have performed Istikhaara prayer about this matter, then the result will be nothing but goodness whether or not he marries her, Inshaa' Allaah.
This is indeed the benefit of making Istikhaara as some scholars say, because in reality it is a supplication made to Allaah so that He will choose for him that which is good.
However, it is difficult to determine whether or not one's heart is comforted and content after the Istikhaara. Before making Istikhaara a person may already lean to such a matter and think that this is due to the Istikhaara, so there is no harm in acting against one's feelings, because one usually feels at ease after performing Istikhaara prayer.
The same thing applies to a dream, one usually feels at ease if he sees a dream after performing Istikhaara, but one can not rely on it [dream]; rather this depends on the extent this matter has been made easy after the person has made Istikhaara.
Finally, it should be noted that if the parents do not agree to this marriage, then he has to obey them because it is an obligation to obey them as long as they do not order him to disobey Allaah. Whereas, getting married to this woman in particular is not an obligation on him. It might be that Allaah will grant him a woman better than her instead.
Allaah knows best.