custody for step-parents

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Can a step-parent get full-custody of the childen? Here si the sitaution: wife had 2 kids from a previous relationship, they are now 4 and 5. Their father is passed away. step dad raised them for over 2 years. In the mean time, mom cheated multiple times, got pregnant twice by different men (not step dad), and got charged with child endangerment (got probation), felony destruction of property (got probation), and 2 counts of felony assault 2 stemming from statutory rape charges on a 13 and 17 year old (going to jail in a couple of weeks. She is out -of-control, irresponsible, and stepda is leaving her. He wants to file for divource and move on but he fears for those little girls. He loves them but is afraid he has no rights to claim custody, though they would be far better off with him than with that deadbeat mom. he is going to ask her for power of attorney and legal gusrdianship while she is in jail (she doesnt know he will file for divourse yet) but really wants to file for full custody. He has no money for a lawyer and is in need of any inofrmation on how to maximize his chance of success and how to proceed with his petition. Can anyone help? are there any precedent? any laws that cover such situation?