My brother is going into 7th grade and he still doesn't know how to spell simple word like people. What can we do to help his vocabulary. We don't want him to go into high school and be made fun of?

Spelling is hard for many students.  Your parents and his teachers should take the lead in helping him.  Being a supportive kind sister is plenty for you to handle.  😊

Here are a couple of links for your parents to read.

https://childdevelopmentinfo.com/learning/tips-for-helping-kids-and-teens-with-homework-and-study-habits/spelling/

https://www.spellzone.com/unit02/page1.cfm

They may also wish to have him tested for learning differences, such as dyslexia.  The knowledge gained from testing can guide his learning program.

There many ways to help him practice his spelling .. Phonics is a skill that requires drills and practice.

This is not a unique problem .. Sadly a lot of students fall through the cracks (so to speak) and can make it right through to high school graduation and remain illiterate.

There are tutors and online programs that can help. He should also have his eyesight tested as well as being tested for a learning disability. Just remember, and remind him, that many people with learning disabilities have higher IQs.

I was also a terrible speller.  Actually still am but thank a computer person for spell-checker.  I problem was that I learned to read before I began school - the see and say method.  As a result, I just kinda tap-danced my way thru the phonetics part of the curriculum.  I learned early that if I was not ABSOLUTELY sure, to look it up. 

However, the checking for disabilities is important.  A friend was a specialist in the field, and worked for a local community college.  She said it was amazing the number of students who had struggled all thru school who would come to her because they couldn't keep up in college.  Turns out that all along they had a disability that could be compensated for years ago.