The records online are from the General Register Office
https://www.gro.gov.uk/gro/1879 June quarter Blackburn
Cecil Percy Bell mmn White
Lancashire BMD records are from local register office sources.
There are two entries - both with the same reference which show mmn as Yates and White.
Presumably the birth registration (I can’t see one on online trees) would show mother as Mary White formerly Yates (or similar wording).
Perhaps Edward meant ‘half brother’ but I think ‘step’ was used more widely then for various relationships.
I am glad you have ordered the certificate and hope that it is helpful.