If a child is born with certain weakness or disability whose fault it is?

Nobody's fault. It's simply destiny....praarabdh. I can understand how people, especially in-laws....relatives can be cruel and making your life hell. But, even though easier said than done.....you have to become a wall and just concentrate on your child. Remember You are the child's Only Life. It's the child's birth right. You should shower all love and care. Pretending that it's just the child and you for each other in this world.