Yes, you can deduct supplemental dental insurance through a different provider if you itemize and the total expenses for you, your spouse, and your dependents exceed 10% of your adjusted gross income (7.5% if you're 65 or older as of December 31, 2016). The deduction is limited to the expenses above and beyond this amount.
You can claim medical expenses paid for your mother in law if you provided over one half of her total financial support in 2016.
You may also claim your mother in law as your dependent if:
- You provided more than half of her support in 2016
- She earned less than $4,050 in gross taxable income. (Social Security income generally doesn’t count here.)
- She lives with you 365 days in the year or is related to you. 32
- She isn’t a dependent on someone else’s tax return.
- She isn’t filing a tax return with a spouse (married filing jointly).
- She is a U.S. citizen, resident alien, national, or a Canadian or Mexican resident