My step daughter lives with me with her son. She has left to "work" and left her son here with my adult daughter babysitting. She has not returned in 5 days now. Can I get emergency custody of my step grandchild?

If she is only gone 5 days, and everyone agreed to "babysit" for her while she is gone, no,

What was the agreement, you can't agree to baby sit and then go to get custody.

No, but you should try to get temporary guardianship in case of an emergency.

Do you have a way of contacting the mother?

... Can i get emergency custody of my step grandchild?

What is the emergency? You have de facto custody now. Why do you figure you need an emergency order to make it de jure?