Why does the poet feel Aunt Jennifer will never be able to escape from her oppression?

Aunt Jennifer is a married woman whose hopes and dreams get translated into her beautiful handiwork, without getting realised. The institution of marriage, symbolised in the "massive weight Uncle's wedding band", has kept her from ever fulfilling what she had wished for herself. And even after she is dead, those dreams remain unfulfilled, and all her suffering unhealed. She would have died, defeated ("mastered") by the situation that life had put her in.