Watched it today having never heard of this case.
The simple answer is it's a mental illness - that's why Beverley Allitt is still in Rampton. I think if they (anyone) could prove that Meg's liver disease and heart problem (her ultimate cause of death) was caused by being unnecessarily over medicated by her mother then you'd be talking. But still, Jean is unlikely to face any punishment if the MBP can be proved. The usual sentence for a diminished responsibility plea, which is likely what this would be and accepted by the prosecution, if the evidence from experts is strong enough, is a hospital order. But the subject of the 'threat' (Meg) is no longer here so what would be the point? Although Jean could have found a new target amongst her over 60's followers
A very sad case. I think her sister must feel so much guilt because basically she ran away and left Meg in Jean's clutches. So many WTF? moments though.