So does the mother know where her baby is at? Is there a chance she's going to come back for the kids today? Are we talking hardcore drugs or something like pot?
We went through something similar with my sister so first, I'm sorry.
If the mother knows where the baby is, what is she planning on the aunt doing for food? Did she leave breastmilk or formula?
If we're talking about hardcore drugs and you believe the baby to be in danger either from exposure or neglect when mom is high, given that it's a Sunday, I would suggest your aunt take the baby to the emergency room. Let them know you believe the mom to be breastfeeding while on XYZ drugs and are concerned about the baby. They will get a hold of social services, drug test the baby, and start the process.
If we are talking about something like pot and there isn't anything else going on (severe neglect, etc.), I would handle it a lot more gently and if you feel involving DHS is necessary, do so in the morning.
Mama to DS (6/07) , DD (6/09) , and DD (07/12) ..