My brother's prospective wife has done istikhara and it came out positive.
Does my brother have to do the same and what if it comes out negative? Can they still marry each other because they like each other a lot?
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Please take a look at the Related posts which appear on the right hand and search on the site by tag istikharah.
Istikhara is a known and recommended sunnah of our Messenger (See for example in sahih al-Bukhari)
Of course your brother is encouraged to do istikhara as marriage is a big matter one should try to ask Allah for guidance at. What the result of istikhara for your brother maybe, i can't tell, but if it was positive he may feel good about the marriage. If it was negative it is still his decision whether he wants to marry that girl or not. And he is free to repeat it.