Hey guys
So I’m studying level 19 (Family) in the A1 German course and I found something confusing
There’s this sentence:
Er ist mein Bruder
And this sentence:
Sie ist meine Schwester
Now the memrise Mem says that it’s mein Bruder and not meinen Bruder because nouns after to be are not objects, if this is so, then how come we change mein to meine in the second sentence?
Just like adjectives, the possessive pronouns have to agree with the gender of the noun.
der Bruder is masculine, so it has to be mein in the nominative case.
die Schwester is feminine, so in the nominative an e has to be added at the end of the ‘normal’ form -> *meine Schwester.
This website explains the whole concept quite well, I think.
Unfortunately, while Memrise is excellent in teaching vocabs, it’s not particularly good at teaching grammar, so using it as the only website to learn a language might not be enough. I’d suggest using Duolingo as an additional tool to Memrise. Otherwise there are also plenty of blogs dedicated to teaching German grammar out there. 
Only mein and meine would be used in the nominative case. That is, if something is the subject (who the sentence is about), then it will be in the nominative case. Mein goes with der and das, and meine goes with die (feminine and plural).
Therefore:
Mein Bruder ist 14 Jahre alt. (der Bruder- brother)
Meine Schwester ist 13 Jahre alt. (die Schwester- sister)
Mein Buch ist neu. (das Buch- book)
Meine Eltern sind alt. (die Eltern- parents)
The list for gender and numerus is:
‘Mein’ for singular masculine/neutral noun
’Meine’ for a singular feminine noun
’Meine’ for a plural masculine/neutral noun
’Meine’ for a plural feminine noun
After sein follows nominative!
you mean nouns directly after sein, and only when used after sein as “copula”, as copulative verb. German has in fact three verbs that can function as copula: sein, werden, bleiben, scheinen, heißen