I think they could get away with saying that Pat was born one of a pair of identical twin girls, out of wedlock.
Her strict grandparents demanded that both girls should be adopted, but seeing their daughter's distress eventually softened their hearts and allowed Pat's mum to go back to the children;s home to get her babies back, but too late. One baby had already been adopted and the children's home would not give an address, as they said the deed was done. Pat's mum took home her remaining daughter - Pat.
Pat grew up never knowing she had a sister. The sister is now tracing her roots and comes to Walford searching for her long lost twin ....
far-fetched, but it COULD work .....