We saw istekhara for marriage and it came a no. But everything about the girl and her family is good. We saw it first because there were some misunderstandings between my brother and his fiance. Can we see istekhara again after few days? if yes after how many days?

Scholars say another istekhara can be done when circumstances change. If the circumstances are the same, one should not violate the istekhara. Allah sees what we don’t see. So if nothing has changed I don’t recommend you reconsider it. If some circumstances have changed then you may pay sadaqa and have it redone.