If it's true, that Bertie is Peter, Lord Hexham's heir, then it means two things 1) Peter's cousin is related through her mother (as she has a different surname) or is widowed. 2) As Peter's cousin - a closer heir than Bertie - does not inherit, Brancaster Castle passes, like Downton, through the male line only... meaning that Peter's surname, like Bertie's, is Pelham too.