Mine are kept at the same temps and I have never had a problem with ris before. Maybe you have a bad strain?
Please remember this is my opinion...
But if it were me I'd co2 mom and the babies and any babies she's had in the past and go buy a juvenile 1.3-1.5. Then restart my colony. If you has a history of ris in the group through inbreeding it will amplify it. (i think)