Like many of the major women in this novel, Pip's (much older) sister is not a pleasant person. She is unkind to Pip, resents his presence, and abuses both Pip and her kind-hearted husband, Joe. She frightens Pip. Pip and Joe bond into a closer relationship because they both have to lean on each other to withstand her.
Pip says of his sister,
I had known, from the time when I could speak, that my sister, in her capricious and violent coercion, was unjust to me.
Pip knows that his sister's temper tantrums, and the way she slaps and hits him, are wrong. He says it contributes to making him timid and sensitive.
Pip is almost as afraid of being caught by his sister while he is stealing food and supplies to help the convict, Magwitch (who is hiding in the fens), as he is afraid of Magwitch. Mrs. Joe provides a stark contrast to people like Joe and Magwitch. Although she is Pip's flesh and blood, men who have no blood relation to Pip act more as real parents to him, nurturing him and caring for him.