Watched the whole series and thought it was FAB!
From the last episode i suspect that Mrs S was/is Sarah's genetic mother and one of the original scientists. She certainly looks like her.
The birth mother was presumably just implanted with fertilised embryos and was trying to warn Sarah about Mrs S.
Maybe Mrs S hiding Sarah all her life was all a lie and she's been monitoring her all along? Or maybe she had a change of heart like anonacfr said upthread.