Esperanza, the narrator of The House on Mango Street, explains in the sketch "Boys & Girls" that 

Nenny is too young to be my friend. She's just my sister and that was not my fault. You don't pick your sisters, you just get them and sometimes they come like Nenny (p. 8).

In spite of this unflattering description, Esperanza goes on to say that she feels Nenny (nickname for Magdalena) is her responsibility as her sister is younger. In "Laughter", Esperanza further comments on the relationship with her sister saying that they don't look like sister "not right away". Yet, there's more to their common bond than physical appearance, the sketch explains, as they often share the same impressions on things. In "Hips", the narrator thinks her sister is saying stupid things but she  agrees with Nenny because she is her sister and she feels a duty of protection towards her (p. 50). Compared to Esperanza, Nenny seems to fit better in the traditional roles cut out for Mexican-American women. In "Beautiful & Cruel", for example, the narrator tells Nenny's thoughts on marriage

Nenny says she won't wait her whole life for a husband to come and get her (p.88).

Nenny wants a good marriage, not like their friend Minerva's sister who "left her mother's house by having a baby", but all the same her desire to marry fits in with the tradition. In contrast, Esperanza announces that she has decided "not to grow up tame like the others who lay their necks on the threshold waiting for the ball and chain" (p.88). Contrary to Nenny who has a pretty appearance and can use that to be pleasant for men and obtain what she wants from them, Esperanza says she has begun her own quiet war:

I am the one who leaves the table like a man, without putting back the chair or picking up the plate (p. 89).