why using "til" and not "av"? I thought "av" meant of?
i am wondering the same thing!
"til" can also mean "of" but I have yet to reliably work out which to use in any given sentence.
Is there a difference between kusine and søskenbarn?
Yes, "kusine" can only be used for female cousins.
kusine = (female) cousin
fetter = (male) cousin
søskenbarn = cousin
"Datteren til ei tante er en kusine." may also be right, isn't it?
It is correct, but a little strange. Most people who use the feminine article "ei" would do so for all feminine nouns and also use the "-a" ending where possible. "Dattera til ei tante er ei kusine" would be more natural.
Ok, I just try to mix it up so I know different versions. I think that is the part of the language I'll only get a grip on when I start reading Norwegian for reals.