Yes, it's just that it's (from what I understand) mandatory to use sia when the possessor is the subject
Right; otherwise, it would be interpreted as being about another female person's brother. My daughter does not like her brother (i.e., the brother of someone other than my daughter).
That's correct, nor with vi.
It's not like Slavic svoj but rather like Scandinavian sin, only used in the third person (he/she/it/they).
Whose brother doesn't she like?
Her own? Then use "sia".
Some other girl's brother? Then use "ŝia".
The most likely interpretation of the English sentence is that it's her own brother that she doesn't like, so "Mia filino ne ŝatas sian fraton".
But without context, it's also possible that your daughter (Jane) doesn't like her friend Lucy's brother -- Mia filino ne ŝatas ŝian fraton.
Or what about this relatively plausible example:
- Ĉu vi konas Sofian? Alica ne amas sian fraton, sed Sofia ja amas lin.
- Ĉu vi konas Sofian? Alica ne amas ŝian fraton, sed Sofia ja amas lin.
Both translate to:
"Do you know Sofia? Alica doesn't love her brother, but Sofia does certainly love him."
But in sentence 1 it's about Alica's own brother (sia refers to something belonging to the grammatical subject), and in sentence 2 it's about Sofia's brother (ŝia refers to something belonging to another person than the grammatical subject)
Why is it useless? The English sentence to me is not clear. Is it her own brother she doesn't like, or another female person's brother?
English manages fine without it. Russian has it for all persons (you say the equivalent of "Mi legas sian libron" for "I am reading my (own) book"). Esperanto has it only for the third person.
Yes, it's not necessary. Neither are definite articles, I suppose - many languages get by without them. But English and Esperanto have those.
"Sia" is there in Esperanto and so it has to be learned.
Imagine a conversation about two individuals of the same sex.
Here are Adamo and Antono. Adamo loves his girlfriend. Antono doesn't like his girlfriend.
Q: Whose girlfriend does Antono not like? His own or Adamo's?
A: You don't know.
Now let's do the same in Esperanto:
Jen Adamo kaj Antono. Adamo amas sian koramikinon. Antono ne ŝatas sian koramikinon.
Here it's very clear that Antono in fact does not like his own girlfriend. Change sian to lian and suddenly it's Adamo's girlfriend that he doesn't like.
This isn't much of an issue when you're talking about people of different sex, as ŝian/lian will give it away, but this is not a completely outlandish situation.
Though it's not guaranteed that it's Adamo's girlfriend that he Antono doesn't like. It could be Arturo's girlfriend -- from "lian", you just know that Adamo likes some other male's girlfriend. (It's probably Adamo's because he's the only male mentioned recently.)
Absolutely, but you'll rarely have a completely new character introduced simply by “li”, I think it's fair to assume that if the person has not been mentioned within a reasonable timespan, it's probably not that person that's being referred to.
Some other languages don't use it. But then there are features of other languages, including English, that Esperanto doesn't use. If you cut out all grammatical features that weren't part of every single language then you wouldn't have much left to work with. Sia affords extra clarity with less words/repetition.
I agree. Sia does prove useful in some cases where there might be ambiguity.
In my opinion the confusion comes in with how it is explained in this lesson. There is no mention of sia's particular usefulness in avoiding ambiguity. The example sentences in this chapter also fail to show it clearly. Instead, the examples focus on all the cases where sia would be quite useless. So I understand the frustration. Sia just looks like a duplication of lia and sxia.
It would help a lot if someone could revise the lesson notes to clearly show the value of sia and provide more appropriate examples showing exactly this. Sentences with subjects (agreeing in sex and number) in multiple clauses help a lot in getting the point across.