You’ve done some of your research and here is a link to the rest from ElderLawAnswers.com.
As you’ll see, there is a lot of gray about whether only medical expenses are deductible, or also room and board, depending on the condition. Yours seems to qualify.
BUT the real issue is not that the expenses be deductible to your father-in-law (who likely does not have much income to deduct these costs from!) but whether they are deductible to YOU! Read to the end of the article to see under which circumstances the adult children of the patient might be able to deduct the costs.
And, I would get a CPA to do your tax return –to make sure it is filed correctly because this is going to draw attention from the IRS!