Bone marrow sibling one-in-four?

A friend is having Leukemia. He is waiting for Bone Marrow transplant from one of his sisters. I searched and searched the internet, all the websites mentioned the match rate is 1 in 4 (i.e.: 25%). That is what the doctors told us too. Can anyone tell me whether there is a simple math showing how they calculate that 1 in 4, or point me to any website showing how the calculation is done ? Thanks !