I thought this Sister would make a good character to join the cast of Father Brown as soon as I saw the episode where she appeared, 'The Bride of Christ'. She was really good and stole the episode. So it was good news that they decided to give her a show of her own. It's really too bad they botched it.
Part of the intrigue of Father Brown is that he has a quality foil to work against him (Chief Inspector). Apparently the writers/producers of this show decided that they couldn't do the same with a female lead character, for whatever reason, so Sister B is given free reign and never opposed or questioned. It takes the heft out of the plot immediately. And then the BBC's own affirmative action quotas apparently caused them to cast an actor who has no experience or talent to be the 3rd lead character, with no personality or interesting quality of his own. He's so bad you can actually watch him "acting", which amounts to him reciting his lines, no more. Compare that to the quality lineup that they give Father Brown--true pros like Sorcha Cusack, Nancy Carroll, and Alex Price (he's brilliant, in my opinion), and of course all 3 Inspectors. But now it's clear that the BBC has decided it's more important to be progressive than good so now this is just a disposable throwaway piece of nothing. Truly a shame that the BBC, in its quest to be inclusive, ruined the chance for a strong female lead character to be great. That's called irony.