I agree. Can you tell me how to help my sister earn his respect?

it takes time and compassion.
Roosters are proud protectors and are designed to defend their brood so when Anyone or anything feels threatening they Will attack. It's no different from a parent standing up for their children or partner when they're threatened, or a dog defending it's family from an intruder. Anyone who thinks a rooster being aggressive should be destroyed or abused, is obviously someone who doesn't give a toss about the bird. And definitely doesn't have any respect for them.
Before getting your sister to go out to your roo, assess his space, what's there as a barrier, not for any reason other than for him and yourselves to grow to understand each other. All animals need boundaries, and if there are none of any kind, it allows them to be boss, that forces you as the carer to respect His space but he doesn't have to respect you as the intruder. Create a boundary, whether it be a fence, a harness with tether (these are great when grass is sparse or they fly over fences) let him learn you're no threat by getting down to his level. I've learnt that eye contact is extremely important to a rooster, they Look at you. When they're relaxed and comfortable, you can see it in their eyes, posture and the way they breathe. He'll gently spread his wings airing himself without feeling threatened. When they stand tall and do it, it's a warning.
Take note of the clothes you wear around him, if he starts looking agitated, he doesn't like it, as I said in my reply to you, I wear the same clothes around Rhodey All the time, he doesn't mind some changes but bootcut jeans, it's a hell no!
The first time I had no sleeves around him, he flipped his lid thinking he was being threatened until I picked him up and gently rubbed his comb, they like that by the way. And under their wattles.
Once you're completely aware of how you act, communicate and what you wear around your rooster/s teach your sister to mimic you. Learn your Roo's an the girls chatter, mimic the sounds and say them back. It will take a bit of time, but when you both understand your Roo's, they'll respect both of you.
They are 'one person ' birds tho, so it may not change, but as I said, respect is earned, so you'll have to earn his trust first.. the respect will follow and once you've got it, it'll stay for life
