Should you let your 6-year old sister just chill when they are peeing intead of making them go to the toilet every now and then if you are her 13-year old brother?

Yes. It is good for you to see her pee (not for your benefit, for hers even though you are her brother. Every once in a while, take her into your bedroom or a bathroom, and strip her of all of her clothes. Then just lift up one of her legs, have her show you her vagina, then tell her to let out a nice, long, hot, relaxing stream of her pee straight from her pee hole, which is right abover her sex hole. It is amazing how much pee a 6-year old girl can let out if she just chills, know that her brother is focusing on her vagina, and you are beeing proud that she is relaxing while she pees. Her pee stream may last over a minute! That is a lot of pee from a six-year old girl's pee hole! By the way, I have a 3-year old sister (I am 13 also), and I see her vagina all the time. She wants for me to see her privates.