My sister didnt even realize how often she was letting my mom step in...unless she disagreed with the action. My mom could not win in this situation. It was OK to bath him, feed him, put him down for a nap...but dont raise your voice/issue consequences to my child.
Until she moved out on her own, she had no idea how often my mom actually did help out...now that she had to tuck him in every night, and actually make arrangements for child care upon leaving her home everytime she left...was big eye opener for her. She now knows she took a lot for granted.
You raise some excellent points.
I will bet that when the OP's mom takes her son outside to play, or to the grocery store with her, or puts him down for a nap, or watches him when she wants to go out, or feeds him, that she never complains. I think it's more about convenice than anything else.
Of course it is her son and mom should step back as much as possible to let her be responsible for her own child. But maybe she feels that her daughter isn't doing enough for her son and feels she has to take over once in awhile? If that's the case, the mom should be telling her daughter how she feels about it as well as the OP talking to her mom about raising her son.
Now is the perfect time, since your living at home, to get yourself a college education and a decent job, if you already aren't. You should be thinking of getting out of that house as soon as possible and on your own with your son. And if you don't want to do that... then stay living with your mom and put up with your her behaviour. Your choice.