Ahaha! ChocoBoy got the overall theme, and guessed (with Gamblet correcting) the cousins.

People believe most of us to vile,
This refers to the fact that the Sorceresses are "hunted" in the game, which in turn relates to the innate fear that the average person has.
Yet we are often misunderstood,
Most people misunderstand that Sorceresses are just as human as their are.
Hiding in fear because of that,
Sorceresses live in fear of being found out and subsequently killed from the innate fear that the other humans have.
A Knight helps us remember that we are human.
Edea and Rinoa are the only known Sorceresses with a Knight, it can be assumed that the other.

Terrible is our futurist cousin for her wish,
Ultemicia lives in the future.
Angelic is our current cousin bravely standing,
Rinoa is the current cousin, and the "angelic" part comes from her Angel Wing ability (and her pet bitch-dog, Angelo).
Pleasant is the pseudo-mentor of our current cousin,
Edea is pleasant, we can only assume that she will mentor Rinoa thus she is a pseudo-mentor.
Our ancestor is the only male version we know today.
Hyne is the only known male Sorceress (Sorcerer would be the correct version).

Do you know who our cousins are?
Rinoa (current), Edea (pseudo-mentor of current), Ultimicia (futurist cousin), and Hyne (only male version). Honestly, I did space out on Adel.
Can you guess what we are called?
Sorceresses is what they are known as in English, in Japan they are called "Witches".

Cousin is used broadly.

Edit: Knights are more then likely linked to the Sorceress romantically, however seeing as we only have two known proper Knights... .