A landlord will almost always owe capital gains tax, selling to a relative is not an exception.
If your mother is willing to sell below FMV to transfer the property to you, it might be possible to make it a gift, but there will be depreciation recapture to pay and if she doesn't pay it, you must, even if you live in the home as your main residence and will eventually qualify for the exclusion. But if your mom needs the money, you can't just "buy" the property below FMV and then "give" her some extra money, that's a sham and the IRS won't allow it.
You may want to take your situation to a professional tax advisor, tell them what you want to do and they can tell you the best way to do it.
