I have one older brother, but no sisters. (sounds more natural)
I have one older brother, but I don't have any sisters. (also natural)
... but I don't have a sister (Creo que es correcto, pero suena un poco raro).
He is *in* 6th grade.
Si "17 de September" es el nombre de la escuena, se puede decir "He is studying at 17 de September High School", que es lo que escribiste.
Pero, si quieres decir que tu hermano va a empezar la escuela secundaria el 17 de Septiembre, tienes que decier algo como: "He is studying High School starting September 17th," o "On the 17th of September, he's going to start High School."
He likes *to* read the newspaper ....
She likes *to* cook for us and watch romantic[ ] movies on weekends ....
I love them because they are *very important people TO me*
[ ] = sacqué algo de lo que fue escrito.