How could I find out if it is "her" or their"? Context or making it capital (Ihere)?
Aaahh... Is it nominative because "you can put the equal sign"? (I remember seeing a tip like this at some point)
yes, the technical term is 'predicate nominative' but the idea of an equal sign captures its use very well.
Think of an equal sign when the verb sein is used. Sein, when used alone, describes a state of being - I am/was, you are/were, etc. There is no action being performed upon a direct or indirect object...it is equating one thing with another.
doesnt Ihr means " you plural " , ? so it seems like " you are your daughter !
Yes and no; 'ihr' (no capital) means 'you plural' as a personal pronoun. But 'ihr' as possessive pronoun means 'her'. And it gets the same ending as 'ein'. And 'Ihr' (capital) means 'your' (formal) as a possessive pronoun. So if Ihre would have been with a capital your translation would have been right, although it would be a biological miracle.