Yes, as another poster said, turn off your phone when there is a family get-together that you don't want to attend. Your mother is still treating you like a child, and so far, you are responding the same way. Now that you are an adult, you need to establish your own, personal psychological boundries. Your mother may never fully accept that, but you do need to do it. Before you start on your new program of communication black-out, do tell your mother that from now on, when you don't want to attend a family function, you won't be going, and will be turning off your phone until it is over, so she should not bother calling you then.
I guarantee that your mother will be ticked off, but she is being very rude. You have right to establish your boundries, but she will continue her manipulative behavior. Such people rarely stop doing that. My mother never stopped.
Grow a spine, and politely stand up for yourself. And remember, we on the spectrum are all:
A Different Drummer
If a man does not keep pace with his companions,
Perhaps it is because he hears a different drummer.
Let him step to the music which he hears,
However measured or far away.
--Henry David Thoreau