Broo the treatment that MSB has does not directly treat the leukemia cancer.
It's for a complication of the cancer treatments, which suppress the immune system and make it vulnerable to Acute Graft Versus Host Disease (aGVHD). This occurs when the donor's T cells (the graft) view the patient's healthy cells (the host) as foreign, and attack and damage them. Graft-versus-host disease can be mild, moderate or severe. In some cases, it can be life-threatening.
This latter disease is what has had around 70% mortality rate. The treatment calms the system and stops the immune over reactions.
The first thing I'd be doing is identify and research that particular form of leukemia he has, if he has it, after correct diagnosis.
Contact the Cancer Institute in your state and go from there. Internet research can be confusing and misleading if you aren't equipped with the precise diagnosis. You need to stick with the reputable sites, eg. Cancer Council, Mayo in the US, the government websites etc, You're in the fact finding stage first.
For instance, last year a close friend was diagnosed with lung cancer which overall has a very low survival rate. But I didn't have the exact form to do any research. She took the view of "not wanting to know" so only her husband and a close friend who was a nurse, took charge and kept the rest of us in the dark.
I had to wait. It turned out, miraculously, that she had the least aggressive form.
So although the tumour was big, (which she told me about!) it hadn't spread to the nodes. They operated and she is in the clear. It was somewhat alarming to only have part of the story, and I knew most of the forms of lung cancers are very aggressive.
My point is, get all the facts and then you can do the research. Calmly.
You're flooded with a lot of emotions and it's hard if you don't have the facts so you can work out the strategy. If he does get a cancer diagnosis, talking to a cancer support group might be helpful particularly in being supportive of your FIL and his family and taking the cues from them about how to assist.
(My friend has recovered and is standing in local council elections). Good luck.
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