You would think though, that twins would be registered in the same quarter.
There are 3 Steads registered in Rochdale in Q/E Jun 1909, Esther, John and William.
Lancs BMD shows that John was from the Castleton No 1 sub district, whereas Esther and William were both in Wardleworth sub district.
No Steads are on the same page of the register as each other though.
If this was me searching, I would firstly obtain William's birth certificate (as has already been suggested) -- to see if there is a time on it.
Or you could risk sending for Esther's death certificate - just to confirm her parents - that would definitely clarify it.
Regards June