sister
The word 'sorella' is used in Italian to refer to a female sibling, just like the English word 'sister'. It can also be used to refer to a female cousin or a close female friend, similar to the English use of 'sister' in some contexts. However, in formal or written Italian, 'sorella' is typically only used to refer to a female sibling.
My sister is doing her homework.
In this sentence, 'Mia sorella' means 'My sister'. The word 'sta' is a form of the verb 'to be' in the present tense, and 'facendo' is the gerund form of the verb 'fare', which means 'to do'. 'I compiti' means 'the homework'.
Luca's sister is very beautiful.
In this sentence, 'La sorella di Luca' means 'Luca's sister'. The word 'è' is the third-person singular form of the verb 'to be' in Italian. 'Molto bella' means 'very beautiful'. So, the sentence is saying that 'Luca's sister is very beautiful'.
This is my sister's bicycle.
The sentence 'Questa è la bicicletta della mia sorella' translates to 'This is my sister's bicycle'. Here, 'questa' means 'this', 'è' is the 3rd person singular of the verb 'to be', 'la bicicletta' means 'the bicycle', and 'della mia sorella' means 'of my sister'. In Italian, possession is usually indicated with 'di', but when used with the definite article 'la', it changes to 'della'.