Because you are not at least 65 years old by the end of the year, you apply the 10% threshold to all of your itemized medical costs (which include costs related to your mother). If she claimed them on her own return, she could apply the 7.5% threshold.
A written determination issued to a taxpayer by the IRS that interprets and applies the tax laws to the taxpayer’s specific set of facts. A letter ruling advises the taxpayer regarding the tax treatment that can be expected from the IRS in the circumstances specified by the ruling. It may not be used or cited as precedent by another taxpayer.