I see two likely scenarios.

She may be his sister, but is QAnon herself and is trusting the plan of military action by sting operation, knows her brother isn't in military, but knows if she calls it now, he probably won't be revealed by anyone official as being part of the sting, and will be believed and heralded a hero.

Or, she may be his sister, but is QAnon herself still and trusting the plan, but believes her brother is in the military, despite what he tells her, because she thinks he wouldn't be allowed to say anything, and she deludes herself of this because she couldn't bare to believe that he actually was reckless enough to lead a charge into the Capitol building looking like such a fool and giving QAnons such a bad name. She thinks because he didn't tell her anything and was recording the whole time, he wouldn't be charged for what she figured out on her own.

Never underestimate desperation.