because in french there are two kind of words ; maculin " for a mal " and feminin "for a female".
here are some examples :
masculin féminin
l'homme la femme
un père une mère
le can replace un and la can replace une
for "l'ami", "l'amie", "un ami" , and " une amie". we can say all of these. because i told you that "la" and "un" is completly the same. it's about the beauty of the meaning.
ah! now understand your question.
for "des soeures", "des frères", "les soeures" and "les frères"
it's just like i had said before.
des soeures and les soeures have the same meaning.
les frères and des frères have the same meaning also.
it depend on the sentence.
we use "un", "une", "des" when we talk about something or someone for a first time.
and we use "le", "la", and 'les" when we talk about something that we had mentioned it before .
i hope that is helpful for you.