Luke, du er mitt barnebarn
does barnebarn only mean grandchild or it can also mean grandchildren as in barn (s) -barn (pl)?
It's the same as with "barn" in that it can be both singular and plural.
In this particular sentence it's clear that it's singular, as "mitt" only goes with singular neuter nouns.
Are there more terms for my daughter's son, daughter's daughter, etc? Also, for sister's son, brother's daughter, etc. Takk!
They exist, but Norwegian mostly use other words as nevø/niese, barnebarn, oldefar/-mor