SIL leaves 14 month old alone and she goes out because we are home.

She lives with us and lately she gauges if we will be home and leaves at night. Sometimes it’s just to get gas or something but honestly I’m not sure the amount of times. At times I just go check the locks at night before going to sleep and realize that she’s out. She has stopped asking since my husband says “no.” She just assumes we will take care of the baby if anything happens. Is this legal? I’ve also witnessed her leaving the baby in the high chair unattended for 20 minutes.