させる - you are making someone do it/let someone do it
させられる - you are forced to do something by someone
される - someone has done something to you
わたしは いもうと に ( ) させる ー I make my sister (do something)
わたしは いもうと に ( ) させられる ー I am forced to (do something) by my sister
わたしは いもうと に ( ) される ー My sister (does something) to me
Mmmmm,,,, it's very hard to explain! I hope it helps!