Without looking it is difficult to say something.
1. are the dead bees workers or drones?
Maybe the hives decided to get rid of the drones and slaughtered them.
2. Somebody sprayed a nectar source with perstizides?
3. Robbery from other hives?
In my area to get rid of the drones is just starting.
Very early this year but not unusual.
Sorry for your mother!
I will hope the best for you & your bees.
1. are the dead bees workers or drones?
Maybe the hives decided to get rid of the drones and slaughtered them.
2. Somebody sprayed a nectar source with perstizides?
3. Robbery from other hives?
In my area to get rid of the drones is just starting.
Very early this year but not unusual.
Sorry for your mother!
I will hope the best for you & your bees.