Would it be correct to assume from the word order that the focus of the sentence is on the people going there, as in an answer to the question "who goes there?"
-- or would that be «мои сёстры ходят туда»? I have yet to get a good intuition about how word order influences emphasis.
"There go my sisters" means "Look - my sisters are leaving - there they go". "There" is this word-order is the same kind of token-word or place-holder used in English to express "There is/are..." The word has nothing to do with location, but just serves as the subject of a sentence because English expects a subject - wants and needs a subject - when a verb is present. The only time you don't see subjects with verbs is in imperative statements, and in colloquial, often short sentences like, "Let's go see the new movie!" "No - saw it already".
OTOH (On The Other Hand), in "My sisters go there", the word "there" is definitely a location, although you need more context to know exactly where "there" is located.
You can't use там here, туда is required because the sentence describes movement towards the location. Там is used to describe an event happening at the location.
Идёт is a singular form, for the plural you need to use идут.
Идти (the verb of which идёт and идут are conjugated forms) is what is called a concrete (or unidirectional, one-way) verb, meaning it describes a single one-way trip from point A to point B. For that reason, a more accurate translation of «Мои сёстры идут туда.» would be "My sisters are going there." Ходить, the abstract (or multidirectional) equivalent, allows the sentence to be more general, to mean that they regularly go there.
By the way, while it's complicated to translate the subtlety of word order in English, the two sentences «Туда ходят мои сёстры.» and «Мои сёстры ходят туда.» don't have the exact same meaning. The first one is an answer to "Who goes there?" while the second one is an answer to "Where do your sisters go?" They're close, but they don't put the emphasis on the same element of the sentence. :-)
Exactly, though as zirkul said higher on the page it can be overridden when speaking by vocally stressing another part of the sentence.
"My sisters walk/are walking there", which could also be translated with "туда" instead of "там": since English has lost the distinction between these two terms, you need to rely on the context.
English used to have thither=туда, but that word is long gone from modern English and is surviving only in idiomatic expressions like "hither and thither".