Lesson 6.4 - Genitive case question

Hallo Nathalie - I have a question on using the genitive case in lesson 6.4 and the ending of the nouns. In the grammar explanation, it says to add an "s" to the ending of nouns the end in -en, -el, -er or a vowel. For the questions: Das ist die Freundin meines Bruders This is the girlfriend of my brother the "s" is added to the end of Bruder But in this question: Das ist die Wohnung meiner Schwester This is the new flat of my sister there is no "s" added to Schwester why is that?

Nathalie - please disregard my post on this question. I found the answer. I guess I missed the part that said feminine noun endings don't change, just the masculine and neuter nouns do you add the -s or -es ending to.

:D