That's certainly a difficult situation. In those situations, it's helpful to reflect on Kamma. That whatever comes to us in our life happens due to our Kamma. It's also helpful to remember that in this world the cycle of human life, birth and death, is unsatisfactory, there is Dukkha. Accepting that Dukkha exists and Kamma, this helps us accept these things. Sometimes it doesn't seem fair that our loved ones contract a disease that it appears was due to the mistake of another. But understanding humans' ability to make mistakes, understanding our humanness, helps us to learn how to have more forgiveness for human beings.
I often use this example: we're trying to be very, very mindful as we walk along the pathways each day, and the one moment we lose our mindfulness there happens to be an insect along the way that we crush. We see that just that moment of lack of mindfulness meant death for that being. Yet there wasn't the intention to kill that insect. So if we look at the intention behind the person who made the mistake, there was not the intention to kill or to harm, they made a human mistake. This is very helpful to reflect on with our loved one, and then go out into the world and use the diffusing technique, universalize this similar situation where people are making mistakes unintentionally that cause the death of others through their lack of mindfulness or through their lack of care. We start to open to the universality of suffering, that helps to bring up Compassion. And then to actually bring up a reflection on Kamma, that each of us follows according to our own Kamma. One way to develop a little more Equanimity towards others and to accept their suffering more is a meditation that is similar to Compassion and Lovingkindness. But instead of actually doing the Compassion and Lovingkindness, we think of their suffering and add the phrase "They are they owner of their own Kamma." We can do this in situations where we accept that each person does follow according to their own Kamma, and that we can't change it.
A little Buddhist story in the scriptures illustrates how even the Buddha couldn't change another person's Kamma, was of a king, and his son who wanted to be king. The son purposely left the gate shut one night when the father was visiting the Buddha and was coming back to the palace. He left the gate shut so his father couldn't get in. His father was quite old at that time, it was a very cold night, and his father died that night from exposure. The son did became king, and later the son wasn't very happy and decided to go and see the Buddha and learn about how he could become happier. He came to the Buddha and the Buddha gave a discourse to him. At that time he changed his mind and decided to become a follower of the Buddha, and professed that he was a follower of the Buddha from that day forth. Yet after he left, the Buddha didn't seem pleased at all, he just kept his face totally without expression. The monks, after the king left, asked him why he wasn't pleased, and he said to his fellow monks, "If the king had not killed his father, he would have become enlightened with that discourse and ended all his suffering. But because of the fact that he killed his father, he couldn't become enlightened. He has to suffer in the realms of existence so much longer because of that action." And so due to the action of killing his father, he was prevented from actually getting enlightened in that lifetime when he had the potential for it. So even the Buddha couldn't change that person's Kamma.
Sometimes when we see suffering in the world we just wonder whether it's fair, maybe we're just not understanding what their Kamma is, and that we may not have the capacity to change it. That's a very difficult one to accept actually, Kamma. It's a difficult one for each of us to accept that Dukkha comes to us. But the great joy about the Buddha's teaching is there a way out of being bound to Kamma.
So using the D/D technique, understanding that all human beings do make mistakes, not just seeing our position personally but seeing how universal it is. We learn to bring forgiveness to human beings. And we learn to accept that the cycle of life is not satisfactory.