Questions & Answers about Moje siostry są bardzo szczęśliwe.
Why is it moje instead of mój or moi?
Moje is the plural form of mój (my) for non-masculine-personal nouns. In the plural, Polish has two groups: masculine-personal (groups with at least one man) and non-masculine-personal (everything else, including groups of women). Since siostry (sisters) is a group of women, it takes moje. If it were brothers, it would be moi.
How does siostra become siostry?
For most feminine nouns ending in -a, you make them plural in the Nominative case by changing the -a to a -y. So siostra becomes siostry.
Why does szczęśliwe end in -e here?
Adjectives have to match the noun they describe. Because siostry is a non-masculine-personal plural noun, the adjective takes the non-masculine-personal plural ending -e to become szczęśliwe.
Shouldn't the adjective after są (are) be in the Instrumental case?
Not here! You use the Instrumental case after być (to be) when you are equating the subject to another noun (for example, 'They are happy sisters'). But when you describe the subject with just an adjective on its own, like szczęśliwe (happy), it stays in the Nominative case.