What can I do to make sure he stays with me?

Your sister may have named a Guardian for her minor child in her Will, if she had one. If not, you should contact a lawyer to initiate guardianship proceedings in the probate court of your county. However, the child's biological father has superior rights to custody and would be the boy's presumed custodian. There are many facts that might overcome that presumption, however, which may leave you in a good position to petition for guardianship. You did not say whether your sister's boyfriend is the child's father, but whoever the father is would have to be notified of any guardianship proceedings.

All you can do is file for guardianship. Understand, unless the dad will be a danger to the child, one way or the other, it will be difficult to keep the child from him. Be prepared for a battle.

The quickest way is to petition the court for guardianship. Once you have guardianship, you can petition the court for custody at a later date. You can always go straight for custody, but it is a longer process, which is why I suggested guardianship first.

You will have to go to court to enforce your desires and that will be an uphill battle.

Your only recourse would be to file to become the child's guardian. In the absence of a guardianship, the only people who have legal custody of a child are the parents of the child, not grandparents or siblings of the parents.