Can I say ”お兄さんは結婚いたしております” instead of ”お兄さんは結婚しております”?
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"いたす" is used for what "I" (the speaker) do when the opponent is superior. It is called "謙譲語". If you're talking about the brother of others, you should not use this.
If you're talking about your own brother, you may say "兄は結婚いたしております".