The rose bush in her mother's front yard is symbolic of Valancy's transformation, physically and mentally. When Valancy lived at home, the bush didn't bloom no matter what she did. Then, in a fit of rage, Valancy cut the bush down, almost to its roots. When she returns to her mother's house to inform her of her marriage, the is surprised and delighted to find that the bush has grown back and is covered in roses. Likewise with Valancy, she is cut down almost to the lowest by the news that she's dying. This spurs her, however, to change her existence completely and she blooms for the first time into a braver, more honest and independent person who is happy for the first time in her life.
The exchange that Barney has with Uncle Benjamin is telling. When he accuses Barney of being a scoundrel who lured an impressionable girl from her family, Barney coolly tells him that he's made her happy and that she had been miserable with them. Benjamin is confounded: it hasn't occurred to him that women should be "made happy". More to the point, I think it hasn't occurred to him- or any of them- that Valancy could be happy in her present circumstances, married to a man everyone disapproves of, living in a small cabin on a remote island, with none of the things which the Stirlings consider necessary for a respectable life. To choose that- and seemingly enjoy it- when she could have married a wealthy, socially respected man (despite the fact she didn't even know him) is such a foreign concept to the Stirlings that they can't accept it, concluding that Valancy must be either mad or bad... probably both.