Why all the Ns in 'mit meinen Brüdern' and 'mit meinen Schwestern'? | 2 minutes of German #23

Why does the word for "my" have an "en" at the end of it in both of the German phrases "with my brothers" and "with my sisters"? And why does the plural word for "brothers", usually just "Brüder", also have an "n" on the end of it

[@Pedram_k]

This is what makes me go crazy 🤯😂