When referring to ones own family, or someone who is not unlike family, such as a lifelong or childhood friend, it can just be said as imouto. However, when referring to another family, you should always add a suffix on the end, such as imouto-san, unless given consent otherwise.
Tsuki threw down her brush in a fit of annnoyance and sat down on the mat, legs and arms crossed. "Onii-chan! Why do we have to do this," she whined. "Can't they do anything on their own?"
Katsumoto walked over to his younger sister and sat beside her on the mat. "Imouto, it's not whether they can or not, it's whether you can. You'll never learn anything without practice!"
"But I don't wanna learn anything," she lamented, staring at the brush and parchment as if it were snakes. "I'm going to be the next Empress! Remember?"
Katsumoto chuckled to himself and placed a hand on her head. "Yes, I am sure you will. But remember, one who is illiterate cannot ascend the throne. You must be both wise and intelligent to take such a high position!"