Blanche most certainly was jealous of Stella. In many ways, she would not want Stella's life. She has little respect for Stanley, and does not understand why Stella is so dutiful to this tempermental man.
However, Blanche's tragic flaw is that she lacking an identity for herself. Stella holds tight to her role as wife and sister, doing her duty as she sees fit. Blanche lost her role as wife when her young husband died, and now she has lost both her job and her home. She keeps trying to fit in and her inner desires and need to be noticed keep her alienate. She tries to create identities. For example, she refers to Stella as her "precious little sister" even though Stella is older. She is trying to put herself into the role as the wise and worldly one, rather than accepting her role as younger sister who may need some help.
Her jealousy of Stella comes out in this line: "I never had your beautiful self-control." Deep down, Blanche regrets her bad decisions and destructive behavior and although she scorns the life that Stella and Stanley live, a part of her desparately wants the stability that is provides.