You need to get an appraisal on the house then have her sell it to you at market value and finance 100% of the purchase price on an amortizing loan at a market rate.
If the loan is setup like this, here is what her income will look like:
5% Interest Payable Monthly
Annual Principal Payment
$90,000 House
$6,000 Annual Principal Payment
Monthly Interest
The annual payment will be realized as an installment sale, so if her basis is $45k, then half of that $6,000 is recorded as a gain. She'll also realize $4,500 for the first year's interest income.
That's the best option you have to keep the income to her low while doing this legitimately. Anything else, you're trying to game the system. Personally, as a hefty taxpayer, I would have a problem supporting taxpayer subsidies (medicare) for your mom while she clearly has assets.