Hi, it may be that Sarah was already on the seen, but it could also be that Isaac had already married again after his first wife's death. I have the death certificate of his first wife Louisa Shorthouse, who was mother to Emily, but cannot find any other record for marriage or maybe another death.
The only other record I have is the birth of Jane Reed Harrison in 1866, whose mother was Sarah Ann Reed - hence the middle name. Apart from these, there are just the census listings. I also have the birth records of all the other children, apart from William. If I could find William's birth, I could maybe find who his mother was, but this one is proving elusive. I have also sent for two certificates which are wrong.
Hopeful !