Probably not.
First of all, you would have to claim your mother-in-law as a dependent. You said you provide more than half of her support, but you didn't say that you claim her as a dependent. To do so, other requirements would have to be met besides paying more than half of her support.
In order to claim the deduction you would have to have paid the premiums. If she paid the premiums, or had them deducted from her Social Security benefits, you can't claim the deduction.
For the self-employed health insurance deduction, the policy would have to be in your name or in the name of your business. Your mother-in-law's Medicare certainly does not meet that requirement, and her supplemental policy probably doesn't either. So this requirement probably kills the idea.
