There is no proof whatsoever. Her census ages were consistent, her place of birth was consistent as Melchbourne, there was no other Packwood family in Melchbourne, she married in Melchbourne. Have you checked the PR to see who the witnesses were? I know, even if they were two of James and Elizabeth's known children, it's still not proof that Mary was their sister, rather than, say, a cousin. Have you checked Stanwick PR on the offchance that she was taken back there to be baptised? If traditional naming patterns were followed Jonathan and Mary's first daughter would have been named after Mary's mother - Elizabeth. Not proof though, but circumstantial evidence.
It will come down to the balance of probabilities - and I think she was probably the daughter and second child of James and Elizabeth. Just can't prove it! Even if you find a Mary Packwood of the right age in a Northants parish you're still going to have the same problem in reverse in proving that she was the Mary who married in Melchbourne otp.
