My child will be starting St Antony's Nursery in the Autumn, but I have been speaking to my friend and whose children attend Catholic School in Wandsworth and the catchment area has been lifted from Catholic Schools, so as long as your sister provides a letter from her priest stating that she regularly attends Mass this should be okay (providing they have space). Even though are closer schools nearer to me, I have chosen St Anthony's as it is a really good school.