I'm thankful that your friend's daughter and your FIL have survived cancer, first of all.
And I'm thankful that you recognize that your fear about illness is taking over your life. That's a huge step. It shows how rational you are, and intelligent, and self-aware.
So, logically the next step is to get some help, to get some tools to learn to handle your anxieties. It may or may not be some medication. My daughter has PTSD from a true medical trauma. We took her to a counselor who helped her realize when her fears were arising, and gave her some coping tools. They included breathing exercises, certain behaviors that she could use, and some medication at first (not long-term). He also helped her to ask herself a series of questions. They were questions like: is this bad thing I'm thinking about likely to happen, or not likely to happen? Is this a rational and reasonable fear, or an unreasonable fear? Example: I'm afraid this candle will burn down my whole house. Think: it's daytime, everyone's awake, the candle is safely burning in an appropriate container in the guest bathroom, where we can see it. There are no children or pets who might knock it over. Therefore, a fire is not likely to happen, and I give myself permission to relax and enjoy the scent of the candle. Another example: I'm afraid this candle will burn down my whole house. Think: it's late at night, the candle is burning near an open window where there is a light curtain. Everyone is falling asleep. The breeze is blowing through the window. Therefore, this is a danger and I must take action. I must blow out the candle and close the window. This is a reasonable fear and I have the tools to handle the situation carefully. Rational and reasonable fears were to be handled appropriately. Unreasonable, unlikely fears were her chance to give herself permission to tell herself that she felt the fear, but it would not turn into an situation where she was crying, stressing, shaking, panicking and shutting down. "I feel this fear but I am in control of it."
So you might apply similar reasoning. Example: my FIL had bad reflux but didn't follow his doctor's orders and neglected to get his treatments. So it's reasonable and explainable that he eventually developed even worse health problems. Example: my toddler scraped her knee, but a simple bandaid covered it. It's reasonable to think that a small scrape, properly cleaned and bandaged, will not turn into bone cancer. Therefore, I give myself permission to kiss her little boo-boo and be done with it.
Don't think that your fears are weird, or that you're losing your mind, or that your FIL's health and a difficult pregnancy have changed your life course unalterably. Of course those situations were hard on you. Now you need a little help getting back on top of things, and that's ok. You need tools, someone to come along side of you and help. Please see a counselor. Don't try to diagnose yourself, simply tell the counselor that you are afraid of health issues and fearful that every cough or bruise will become something traumatic, and let the counselor do his or her job.