His sister moved into his building this year, and our class 8 son says they're 'not related' at school

Our daughter moved up to the senior building this June, on the same corridor as her brother's class 8. On the first day he informed her, kindly enough by his standards, that at school they are 'not related'. She still calls him Dada at home and laughed it off at dinner, but yesterday she mentioned she now takes the long staircase so she doesn't have to pass his classroom. I did something similar to a younger cousin at that age, so I'm not treating it as a crisis. I'm torn between having a word with him and letting them sort it out between themselves. Did this settle by itself in your house, or did a parent have to step in?