i also have deaf cousins and i only know a bunch of signs like a week's day and colors and places but i can still hold a conversation. i know you might think this is just fictional but deaf people are really good at lip-reading when they grow up with hearing people. and just like in this work, they can get a made up sign as long as it coveys the message with the gesture and expressions, that along with just a few signs like colors/days of the week/places to describe things and situations, and maybe a bit of writing down words will probably make you hold convos alright with your cousin and i'm sure that would make him happy.
That make me feeling blessed rn bc I’m deaf and I used my ASL (america sign language). Beside idk which they using JSL or KSL. Ahhhhhhh ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ I love that manga soooo good bc for good example or reason why I have an experience through about that. I used to it when I grow up with hearing ppl uh… at least thank you for making manga ╥﹏╥