WHATS MOST IMPORTANT Being 245(i) eligible means that a person can adjust status despite having entered the country illegally. However the major obstacle is that someone, either an employer of family member must have filed a petition for you before April, 30, 2001.
So for example if someone enters the country illegally in 1991. Then in 1999, a sibling files an I-130 petition for them, they may be able to apply for a green card. Usually they would require a new petition to filed by a USC spouse or child over 21.
This new petition would allow them to apply for a green card immediately. They would have to file a form I-485A and pay a fee of $1000.
The key is the petiton that was filed prior to April 30, 2001 must be "approvable when filed." which means that the relationship must have existed at the time of the filing. A USC may even file for a half-sibling. This is a brief explanation and it is recommended that you contact an attorney to deal with the facts in your situation.