how r^21=-r^12 and r2₂₁
==  r2_​12
 

In the first case ${\hat{\mathbf{r}}}_{21} and {\hat{\mathbf{r}}}_{12}$ are vector. And we know that vector has direction. Now the distance from point 1 to point 2 is equal but having opposite direction. That's why the negative sign. 
But when we take the square, it is actually the magnitudes square, which is a scaler and so they are same.