I hear моя and сестра for the first time on this course. Why is it that some new words are marked as such and some are not?
Well I don't know other languages but in Italian it's "sorella", in Spanish it's "hermana" and in Modern Greek it's "αδελφή"
English: Sister; German: Schwester; Dutch: Zuster; Norwegian: Syster; Swedish: Syster; Icelandic: Systir; Latin: Soror; Italian: Sorella; Russian: Sestra; Polish: Siostra etc.
Portuguese irmã, Spanish hermana and Catalan germana (as their masculine counterparts) come from the word germanus, germana, an adjective that the Romans began to add to the words frater and soror (brother and sister) to denote that they were talking about biological siblings, not adopted ones (adoption, even of adults, was very common in Rome). Hence, frater germanus and soror germana, that those languages reduced to the second element.
Actually it is derived from *sestra in Proto-Slavic which is the unattested, reconstructed proto-language of all the Slavic languages.
Сестра has another meaning in Russian - nurse
Just as in English "Sister" used to be the correct official form of address to any nurse (who was not in charge of a ward, that was "Matron", from the Latin for "mother:). Now that nurses can be men, the titles have changed. I believe that the origin of this lies in the fact that the first hospitals were run by nuns; the religious titles were preserved, even for secular nurses.
So, the uses of "моя" "мой" depending on what gender is the noun, right(?)
If I were a girl (feminine), then the sentence "Он мой брат" is still correct? or should I change it to "Он моя брат"?
The adjective declines based on the gender of the object it is modifying, NOT based on the gender of the person using it. So it would 100% have to be мой брат regardless of who's talking.
Children have a special books with stressed letters, and adult people have a knowledge through practice experience.
Its not hard for me, my mom talks russin all the time i want to understand her better