Great question — answering as the UphillTech team:
I believe Robert could be the son of both James & Zilpha - It’s very feasible given their history I think there reference in the episode where James meets Robert, & gives his guardian money, at the end the words in the letter Zilpha sends to James. “I hope you can keep the secrets of the past buried.. buried in a deeper grave.. “ are a indirect reference to Robert. later in the series (season 1) There are other hints like; “I recognised the depth of our sin.” The fact that Robert is hidden away also adds weight to this with James suggestion to Robert’s guardian; “Keep some money saved, in case he grows up to be rash.. like me..” not sort of suggestion I think would come from a sibling.
Full answer by Lauren Monaghan: https://movies.stackexchange.com/a/131364 (CC BY-SA 3.0).
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