Make a list of all the entertaining nicknames that you can think of for your sisters, like smelly, stinky, dirty....that's just the general idea. A good one will work in other details about her, too, or her name, something to make it stick. They both need derogatory nicknames that are catchy enough that other people in their life could pick them up. That's a woman's nightmare. And doing things like that are how women fight with each other. They spread snide, behind the back rumors that are crafted to erode social status.
Really? Is that the best he can do? When girls in high school annoy him, call them names? When girls in college annoy him, call them names? Women at his first job annoy him, call them names? Women in the boardroom annoy him, call them names? His mother or his own daughter, annoy him should he just cal them names? Really???
I see the opportunity here for him to learn THE SKILL of focus and taking responsibility for where he chooses to place his focus, his time, attention and energy.
If he allows himself to indulge in the pettiness of calling names or attempting to control anyone's insignificant behavior, to me that is a poor investment of his energy. How many minutes does he spend distracted and irritated maybe 2, multiply that by say give or take 5 times a day, times 365 days, times a lifetime of another 60 years. By those numbers he will then SPEND 3650 hours of HIS life, into caring about MEANINGLESS stuff others say or do. ONLY he can make what they say or do significant or not. Only he can decide if it's worth spending part of his life on.
I personally believe he is better served INVESTING/spending his time, energy and focus on what truly serves his goals and greater mission, whatever that is for him. The sooner he practices not being bothered now, the easier it will be through his life. Like any muscle, it get's stronger with use. True focus is a powerful capacity. It's not for the weak.
If one is concerned his sister's behavior is a sign of disrespect of him, I suggest it's only as significant as he makes it.
Imagine a very well built man walking down a street. Some goofball slaps him as he walks past. The built man can just laugh at the man's attempt to get his attention, while the built man continues on his way, giving no attention to the goofball. Or the built man can stop, and call the man out for being disrespectful. They can exchange words that get more heated by the moment, to display their dominance. Eventually as anticipated it comes to exchanging blows and the built man putting the goofball in his place, as it's called, before the built man continues on his way. What is gained? The goofball gained a sense of significance as he was able to garner the attention of the well built man enough to stop, acknowledge and then fight, regardless of losing. He gained the built man's time and attention which is significance to those who feel they lack it.
I believe a man who inherently knows his value, knows he has nothing to prove to others. The built man who simply walked past laughing at the goofball's attempt to get the built man's attention, inherently was secure enough himself, that he did not need to spend his own time and energy, bickering and fighting some goofy guy to prove anything to the goofy guy, other people nearby, or himself. The built man was a man on a mission who found the comedy in the idea that the goofball could believe he was significant enough to derail the built man from his mission. The built man on a mission might stop for a few things of true and meaningful significance that are indeed worth his time, but not some goof on the sidewalk seeking significance by picking a fight.
I say ignore the petty. Learn to discipline yourself to do that through your whole life. Let the petty stay with the petty. It's not worthy of your time and attention.
Have a purpose that inspires you so much that you couldn't be bothered by the trivial behaviors of others. Be a man on a mission. Enjoy the rewards as well.