We don't know any of this. It is all simply speculation about people's motivations.
Hopefully schools have changed Nomad. There is no way that my children, at primary school, would think it was acceptable to make unkind comments to another child such as those made to your sister, or to have disputes in the classroom, without a teacher intervening. Teachers, and indeed the headmaster, have phoned me over much less. If you have a zero tolerance policy on bullying then the number of times kids make unkind or simply personal comments goes down and you don't have to spend all day on the phone., and issues are nipped in the bud long before they actually turn into bullying.
As for the mother's response, I would never phone another parent. I think parents should leave the school to resolve disputes in school. I would speak to my child if phoned by the school, but never to another parent or child. The children almost always make up at some point, but the parents end up having an issue with each other forever.
If my child and another child has a dispute and my child was unhappy about it, I do typically say, find someone else to play with; you don't have to play with X child if you don't want to. They don't play with the kid they had an issue with for a while and then eventually it blows over and they make friends. Friendships in school change over time.
None of this changes at all simply because the OP has got worried about racism and phoned another parent to discuss racism. We don't know exactly what happened between the kids, what the teacher thinks or what the other parent thinks, or if any of their responses are about racism. But plenty of people on this thread are encouraging the OP to worry that this by making it out to be some major problem when kids having a dispute is just one of those things that happens in primary schools.