Mom has little interest in my son

My son just turned 2, everyone that meets him tells me he's so relaxed and well behaved. A sweet social kid. My mother, his grandmother doesn't seem to realize she's missing out. When she does finally decide to drop in for a half hour on her way home, she spends most of the time with my son- talking about his cousin, her first grandchild. He has an older brother who isn't my brother's child, my brother doesn't act that way but my mom sure does. She criticizes an 8 year old boy like he's an adult. And says "I wish I could get a picture with just my 2 grandsons" meaning her first and my son- not the older half brother. My brother is the youngest of 5 in a blended family and spoiled growing up, I mean he was given four wheelers, phones, name brand clothes, a brand new never owned vehicle for his first car, all while I had a progressive disability that never got treated until I moved out on my own and got my own doctors.

Now my son is being treated the same. She gives him things the other boy likes and says "I heard chocolate covered raisins are your favorite" ...my son just turned 2- he's never had them. Sure enough, I asked my sister in law what my nephews like- chocolate raisins. At my son's birthday party, my mother went on and on about my brother, fine. But when it came time to "help" I told her she could fill my son's cup... she never did. I guess it isn't a big deal, he wasn't dehydrated but don't ask to help if you're going to ignore the request.

I'm not sure how to handle her anymore. She's dramatic and annoying to be around, she sucks the energy out of every room she's in- always trying to put attention on herself. A checks all the boxes narcissist.

How do I deal with letting my mother go from our lives? Like not totally eliminate her but not care that she doesn't want to get to know my son? How do I be okay with her flaking on visits, rambling on about herself and my brother? If she's not fawning over herself she's whining about her terrible marriage to my father. Idk what to do, any progress I make in therapy is negated after a half hour with her.

99% of people who know her (those that see her act for what it is) understand she's nuts. I as her daughter am struggling to cut off the woman that birthed me.