`relation’ and `relative’
Your relations or relatives are the members of your family.
I said that I was a relation of her first husband.
I’m going to visit some relatives.
The relations between people or groups are the contacts between them and the way they behave towards each other.
Relations between the two men had not improved.
Britain has close relations with the US.
Sooo… seems they’re both right, but because relations has that other meaning, I reckon relatives is more common and acceptable. IMO. So the teacher can stuff off.