My brother is an athiest and my mom is forcing him to a christian school on sundays. He doesn't want to go. My sister says that when he becomes a biologist he can know the other side of the argument. Do you think this is fair?

I am not sure what other side of an Argument he might learn.

Is there an Argument and if so, what is it about? Evolution?

If it is Evolution, it is as much an accepted science as many of your other elementary school science teachings to include the Water Cycle Theory, The Tectonic Plate Theory, the Heliocentric Theory . . .

https://www.sciencenews.org/blog/context/top-10-revolutionary-scientific-theories

If it is abiogenesis, than there are just a few hypothesis but nothing that has reached Scientific Theory as of yet.

While I don't agree with that type of parenting, if your brother is under the age of 18 then he is beholding to your mother's wishes. Tell your brother that he can think of it as a sociological or psychological study of ancients traditions turning from believed truth to myth. He can compare and contrast with other such religions that are now regarded as myth.

It's only unfair if she is preventing him from going to some "atheist" school at the same time.

If he doesn't think that mathematics makes sense and doesn't want to learn it, would you think that it would be unfair to make him learn about it anyway?

At the very least, if his final decision is to not believe in God, then at least he will get a general idea of how---and why---a great number of people believe otherwise.

I certainly don't want to live in North Korea, but I'm glad I took geography and history to know where it is and how it affects me.