Knowing the vocabulary for family members in German helps when you want to talk about your family and tell some funny stories about that one weird eccentric uncle each family has (you know who it is ...if you don't, then you're that uncle!).
In German, the word for "mother" is Mutter, the word for "father" is Vater, but using those to address your parents sounds a little bit old school. Most children call their mum Mama or Mutti, while they call their dad Papa or Papi.
Let's talk about grannies: Children usually call their grandparents either "Oma" or "Opa" - "Großmutter" or "Großvater" sounds a little bit formal.
You could use Großmutter and Großvater when talking to a friend about your grandparents, but you wouldn"t say it to your grandpa or grandma directly - unless you're red riding hood, she does these sort of things... 🙄
Der Onkel & die Tante are also pretty easy to remember. Just like in English, you would refer to "auntie Hannah" as "Tante Hannah", or "uncle Bob" would be "Onkel Bob".
Remembering the German words for nephew and niece isn't that important. Nowadays, many people also refer to their nieces & nephews simply as "Der Sohn meiner Schwester" or "Die Tochter von meinem Bruder".
Der Cousin & die Cousine are more common. Luckily, they're one of the many German-English cognates, so remembering them won't be very difficult.
Sometimes you need to say exactly which brother you're talking about: Your older brother? The youngest son?
How would you say these things in German? Easy peasy: