My DH works in essential services, he and his guys are in contact with the public constantly, everyday they are exposed to the chance that someone might have it, so they do take precautions very seriously, and all his guys want to keep working... DH's company not only does commercial pest control which is necessary, everything from warehouses, processing plants commercial and office building, hotels, restaurants .... to keep the food and supply train up and working and running, as well they have been doing treatments to kill the virus.. Alot of places if someone even sneeze's or coughs they have DH and his guys into treat, as a precaution.
Rules in place, no one comes to the office, when they need products, they are set outside and the swing in and pick it up, mask are a must at all times when working, the minute they get out of the truck that mask has to go on, at time, gloves, shields, googles, Tyvek suits( hazmat suits), DH went so far as to make sure everyone has hand soap, they carry water on all the truck and hand sanitizer in their trucks, it is part of the truck inspections..
In defense of the store that told the sister to come to work... do you know how many people are using this as an excuse not to work, not because they were exposed because they just want lay out for 2 weeks, and want to get paid for it... most places are doing temp check, and are keeping the mask in place. Now I will say I have seen customers without mask, the employee's for the most part are following the guidelines..
I don't understand why the SIL just did not wear a mask into the house, to protect your mom, and stayed away from her -social distancing, and I hope your mom wiped everything down when she left to limit the exposure chances... I hope and pray that your mom will be okay...