First, I'm so sorry about your baby brother, and I hope your mother found the courage to deal with such tragedy.
In the muslim faith, each soul is judged based on what it has earned of good and bad. We don't believe in the original sin. God tells us that our soul at birth is pure and it's us that keep it pure or screw it up. Your brother have a pure soul that had no chance to get tarnished and so he's a resident of heaven.
In christianity with the believe in the original sin, each baby would be a sin that must be purified through acceptance of Jesus. The logic of christianity really fails the babies, the sick, and the mentally ill.