One of the reasons "Sister" is hard to understand is that there are actually three speakers. The narrator appears in the next to last line and only says, "Comment on Stop". The poetic persona is very closely identified with Hughes himself: his sister's name was Marie and it is held that this poem is about her life.

In the first stanza, Hughes' poetic persona complains that a man with a wife and child is paying attention to his sister Marie. The persona seems more distressed at the man's behavior than at Marie's:

Why does he keep on foolin‘ around Marie?
...
But why does he keep on foolin‘ around Marie?

The second stanza introduces the voice of the second speaker in this heartfelt conversation who seems to be an older man. The older man suggests that Marie might be allowing the attention and not getting a "boy-friend / [Hughes] can understand--some decent man" because she wants the nice dinners out, nice entertainments out, etc, that money can buy.

Hughes asks in the third stanza if "decent folks" don't have money too? ["Dough" is earlier American slang for "money."] In the second line of the same stanza, the older man replies that "unfortunately," decent people--in his experience--usually don't have money. [You will have to contemplate situations to see whether you believe that statement was and/or is still true.]

The fourth stanza reveals Hughes frustration and dismay with his sister (as opposed to his earlier emotion focused on the married man) when he protests against Marie's keeping company with a married man, intimating that there must at least be some men with money--decent or not--who are not married:

Well, anyway, it don't have to be a married man.

The older man replies by suggesting that perhaps Marie "does the best she can", in other words, she's accepting what offers she gets: if better men would offer, then she'd choose better.

The last stanza is where it gets tricky. The narrator imterposes and cryptically says there is a comment "on Stop". What is "on Stop," and does it matter? It does matter in that "on Stop" adds to the poem's location, texture, and imagery. "Stop" is usually an abbreviation for "bus stop." This heartfelt conversation is being conducted in a city, in a neighborhood so crowded that a stranger at a bus stop can hear. He can not only hear but is close enough to comment as well. What does his comment mean?

"So does a man" might be looked at as meaning several different opposing things. This is the definition of "ambiguous." Thus Hughes writes an ambiguous last line, another reason (along with the speakers' dialect) the poem is hard to understand.

The comment might be meant cynically to indicate men pick up the best women they can regardless of moral considerations. It might be meant confessionally to indicate men do the best they can at earning money and finding women to share their lives with when women want "some cash". It might be meant sympathetically to express the sentiment that both women and men do the best they can in circumstances that are adverse for all. It might be meant antagonistically to express the sentiment that doing the best you can means taking advantage of gullible people and exploiting situations for your own benefit.

Hughes' tone throughout is a combination of sympathetic concern and knowing realism, so perhaps the ending might be best viewed as the mixture of sympathetic confession.

Comment on Stop,
So does a man.