While I don't know your strength or moms weight,
something to consider might be make a ramp suitable for just the first lower
step so the ramp won't be really long and then you can manhandle backwards
up and sort of wheelie and lower the real wheels down a single step without
great difficulties.

We have a marine plywood ramp here 4.75"h x 72" which equals about 1:12 rise
and anything steeper an elderly person is going to risky rolling down alone.
Rolling down alone was the major consideration here in case of fire. Nailed
2x2 edger except where the door swept across to try to contain it, 2x2 is a
too high or not high enough mistake .. wheelchair hand grips roll on top of
it perfectly lifting the wheels off the ramp, he sits there and freewheels
one wheel wondering whats wrong