I don't believe it's Philpotts. If she's Archie's sister then she's too old - Archie would only be 27 if he was alive; if she was older, then she'd be about 30 as Mrs Patmore has to be about 50ish.
If Lucy is younger than Archie, then she's aged really badly as the actress is, most definitely, above 30.
The actress just looks to old to be Archie's sister. Mrs Patmore did mention she had another sister who died, I believe. She has Kate, Archie's mother, and another sister as she mentions talking with Kate over Archie's death... yet mentions another, previously deceased sister. If she's already dead, then she was probably older (or died in childbirth when young!) and could have an older child.