Your title says "DCFS SENT MY KIDS," but your question implies you did so on your own accord. If DCFS placed them with a relative, and YOU SIGNED A SAFETY PLAN, then your legal options have been obliterated.
DCFS has a reputation for bullying scared parents into signing stuff. Once signed, it's almost impossible to undo, and you must adhere to the terms of the safety plan, no matter how draconian they are.
Take your paperwork to a family law attorney in your area for specific guidance.
your question is really confusing. did you put the kids with your sister or did dcfs? if you did go get them. if dcfs did and you signed the safety plan, then you have to work within the plan to get them back.
take all your papers to a lawyer to answer your questions.