In Islam, such trials erase sins, as long as the person is patient for God. Its one of the trials and tests of this life. Some people react by thanking God for everything no matter what, others would forget all "good" and just show displeasure with God. So in this way also, its a test.
Sometimes such happenings dont make sense to some who dont believe in the afterlife, and see this life as the goal, and everything. So the problem is really with the way they perceive life. Eternal afterlife means this life is really nothing. Logically, infinity - 9999 trillion = infinity. So 9999 trillion here is pretty much a zero next to the afterlife. Its a fact that we sometimes we understand but cant comprehend (at least in this life) , being the humans we are.
And this is in terms of length only, but similarly, quality of afterlife really makes this world just nothing. So when one really believes, understanding his own belief well, hed always be content. In Islam theres so many texts about such things, promising heaven for those who are patient, and some of the texts are specific even, about blindness etc.
Sadly, we humans tend to think about such things only when something happens to someone they know. As if humanity didnt have suffering before that. Who knows, maybe it would end up being a reason for a person to think, observe, understand life, and become much closer to God in the long run and afterlife?
That said, best wishes to you and your family member.