You certainly have a lot going on within your family, & I can see how this all worries & upsets you.
If you can make a long appointment with your GP & talk over all these problems, they can give you good advice. All your family member may have their own GPs they can talk to.
Your GP can help you, &, if you want, they can help find a Psychologist to help you learn how to deal with the stress & upset, & how this is getting you dowm.
Your brother, your father & your wife all will have other specialist Doctors, who they have seen for their conditions. You might have to help to get in contact with some of these & have your brother, father & wife talk to their own Doctors.
Personally, I have had breast cancer, & surgery for that this year, & I know they have people who will be willing & able to talk to your wife about any changes in how she feels about sex.
Try to be patient with your wife. She has been through a lot, & it's scary & stressfu, & she too, surely feels for your brother & father, just as you do.
May I ask, how do you feel your relationship with your wife is going? Are you close & strong, like a team? Or is there so much stress that this is making your relationship & communication between you difficult?
Talk here as you feel comfortable to do. You don't need to even tell us the answers to my questions if you don't want to.