In "The Son's Veto," can you give evidence through which Sophy appears as an incapable poor woman who is unable to shape her life?
You original question read "incapable poor woman who is able to shape her own life." This contradiction must be sorted out as either "incapable poor woman" and "unable to shape her own life" or as "capable poor woman" and "is able to shape her own life." The reason is that the logic implies that an incapable poor woman would not be capable of shaping her own life whereas a capable poor woman would be capable. If the opposite of this was intended, that an incapable poor woman rose up against her circumstances and against all odds became capable of shaping her own life, there is nothing in the question as it was stated to suggest this. Thus, I'll answer "incapable/unable."
The answer to this is "No": There is nothing in the text that presents Sophy as an incapable poor woman who is unable to shape her own life. Hardy presents Sophy as a caring, competent, dedicated village woman who works for a wage as a room-and-board domestic servant. She lives in the village near her parents' house, which has a decorative front garden. Thus she is not poor, neither are her parents poor. They are not rich, but they are not without means for food, clothing, shelter nor without means with which to embellish that shelter with trees and a "white swing-gate."
Further, Sophy is characterized as being able to speak up for herself, to speak her own mind and choose her course without any hesitation or fear. She knows how to correctly value herself since it is "with no surprise" that she sees Sam waiting for her, though for form's sake she pretends to be surprised:
she discerned, without much surprise, the figure of a man standing in the hedge, though she roguishly exclaimed as a matter of form, 'Oh, Sam, how you frightened me!'
She robustly tells Sam without any fear or qualm that he must not kiss her while putting her hand over his mouth to impress the point and following it up directly with an adieu and an exit into the house:
He stooped to kiss her a farewell, for they had reached her mother's door.'No, Sam; you sha'n't!' she cried, putting her hand over his mouth.
She unhesitatingly tells her employer first that she wants to leave her post as servant and then that she would rather not leave her post. When asked if she wants to marry Sam, she unflinchingly says, "Not much," but that it would provide her a home of her own.
After Sophy's fall and after she is nursed in the vicar's household, she realizes her work days will never be the same. She accepts the vicar's proposal of marriage for the same reason she accepted Sam's earlier proposal: it would give her a home of her own and, moreover, a more elevated life than she could ever have aspired to. In Sophy's mind and in Hardy's mind, Sophy is a capable woman who is able to shape her own life. That her life did not turn out the way she anticipated is not to be attributed to Sophy's ability or lack of it.
Yet, if you wanted to show Sophy as incapable and unable, you would take these same instances and interpret them in light of our present day culture and say that because she was a village servant, she was poor; because she accepted Sam, she was unable to shape her own life; because she broke it off due to a quarrel, she was being forced to do so; because she accepted the vicar's proposal, she was blindly following the strongest man influencing her. You might interpret Sophy this way, yet this is not true to the culture, the character, the text or the author.