Risperidone and Aripiprazole are similar drugs for similar condition but their side effects may differ.
Just because two people have the same condition doesn't mean same drug will be prescribed. Risperidone may cause more Menstrual problems while Aripiprazole wont so this may be the reason why your sister got a different medicine.
Next Steps
Consult Your Doctor for any further information
Health Tips
Dont Discontinue Medicines on your own.