Confusion is apt to occur in English in the use of the words "him, her,
it, them; his, hers, its, their", e.g., "John loves his brother and his
children." Whose children, John's or his brother's? "The boys brought to
their fathers their hats." Whose hats, the boys' or their fathers'? "She
gave her sister her book." Whose book? Her own or her sister's?
This confusion is avoided in Esperanto by the use of, the pronoun "si"
("sin"), meaning "himself, herself, itself, themselves", and "sia,"
meaning "his own, her own, its own, their own."
"Si" ("sin, sia") refers to the "subject" of the sentence in which it
occurs; therefore in the sentence "John loves his brother and his son,"
it must be "Johano amas sian fraton kaj sian filon" if it is his own
(John's) son, because John is the subject, but we must say "lian filon"
if the brother's son is meant. "La knaboj alportis al siaj patroj siajn
cxapelojn" means "The boys brought to their fathers their own (the
boys') hats," because "boys" is the subject, but if we mean "the
fathers' hats" it must be "iliajn cxapelojn."
"She gave to her sister her book" must be "Sxi donis al sia fratino sian
libron" if it were her own book, but "Sxi donis al sia fratino sxian
libron" if it were her sister's book. "Li diris al si"... means "He
said to himself," but "Li diris al li" means that he said it to another
person.
"Si", "sia", can only "refer to" the subject, it cannot be the subject
itself or any part of it; therefore we must say "Sinjoro P. kaj lia
edzino tre amas miajn infanojn", because the subject is "Sinjoro P. kaj
lia edzino."
"Mem," self, is only used for emphasis, "e.g.", "Mi mem," my (own) self.