Did you already tell MIL they're for her? If not, you can just make them longer till they fit you. Assuming of course that they fit around your foot. My first socks were for my 9 1/2 shoe size feet but I did them in worsted weight and wear them like slippers. They will go in my winter boots but I only wear them in the winter when it's really cold out and maybe snowy and last winter I didn't wear them at all.
I should never overestimate my basic arithmetic skills.