The word "treat" is one of those English concepts that can mean many
different things and therefore has many different interpretations. For
example my kids like treats (SWEET, CANDY, COOKIE, CAKE).
Now, your sentence "Sometimes people are not treated fairly" seems hard because it is using a passive construction. Sort of like the sentence, "He was shot." Who shot him? You have to establish who is doing the shooting. For example, you could sign "MAN, INDEX-left, SOMEONE-right (bodyshift face-left) SHOOT-(towardleft)"
One way to interpret "Sometimes people are not treated fairly" would be to sign:
PEOPLE ASSOCIATE-(circular sweep inclusive) EQUAL, EQUAL-(reposition) EQUAL-(reposition) ALWAYS? NOT!
That ASL construct would indicate that people do not always interact fairly with each other.
But if you by "not treated fairly" you mean a bunch of things like: taken advantage of, discriminated against, prevented from progressing, given less opportunity...etc. Then you will need to either sign all of that...or use the rest of the discourse to make it clear. You don't just walk up to another person and start signing about human rights issues...there must be a context. To interpret a sentence like that out of context would require several minutes of signing. You could have ten different interpretations and any one of them would be more or less correct depending on the context.
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars