Your question has some facts that need to be filled in. Is there currently a custody order in place? If this is not a divorce situation, has paternity been established.? If paternity has not been established, you should file your petition right away. Until paternity is established, you will not be recognized as the legal father. As part of your paternity petition, you can seek an order for custody.
If paternity has been established, and the order states mother has custody, you will need to file a petition to modify custody. i would suggest that you contact an attorney for a full consultation on these issues.
The answers provided are for informational purposes only and shall not be considered legal advice on your specific fact situation. If you would like information specific to your particular fact situation, please feel free to contact me directly.
As a follow-up to my colleague's response: If there is a custody order in place giving Father and Step-mom custody, then Mother would have to petition the court to change the custody arrangement if she wanted to have the kids. That would require a substantial change of circumstances - something that is significantly different about the circumstances of the parents/kids than how things were when the custody order was put into place. The court will try to determine what is in the best interest of the kids when it decides on custody - and the things you have mentioned about Mother, such as her lack of seeing the kids in the past several years, plus her living situation and the things on her record, will certainly be things the court may consider.
Katherine E. Flood
Indianapolis Divorce Attorney
The foregoing is not intended to be specific legal advise, but rather general information. If you require specific legal advise, you should retain a qualified attorney in your area.