Is "swoich" reserved for the subject of the sentence? If so, then it's just like Norwegian!
Yep, just like norsk, when the object is singular it is like norsk too: ...o swoim bracie - ...om broren sin ...o swojej siostrze - ...om søstera si ...o swoim dziecku - ...om barnet sitt
Can we have 'talks' or must it be 'is talking'? If it were 'often talks about' (quite likely) is there a different verb? Thanks.
It could be "mawia", but "mówi" is also a perfectly good translation. "talks" is accepted.
There is also a verb "opowiadać", which means to tell or talk about sth/sb.
Sentence with this verb would be: "Babcia opowiada o swoich siostrach".
Makes sense, added.
Sorry but why is grandmother wrong now?
It's accepted. Perhaps you made a typo somewhere, then you got a correction which says "grandma". That doesn't mean that "grandmother" was the problem.
Is there any reason we can't have "speaking about" here?
We can, it's accepted, it should have worked.
Can I say, "My grandma is talking about her sisters"?
On one hand, it's not that clear because there's no "I" or "my" nowhere in this sentence, but we can say that it's most likely the speaker's grandma. Alright, added.