Need help with how to get my mother in check

Tink1704

So I have a brother who is 6 years older and had a baby at 18 who he gave up his rights when the mother left him and married another man. My mother never got to know the child until this past Feb when she turned 14. So she is super anxious for a grand baby! Well thing is she is a bit unstable, diabetic, bad thyroid, etc. She almost killed herself last Dec due to her stupidity of being a extreme diabetic and drinking Pepsi and eating really unhealthy while rarely checking her blood sugar. She is telling everyone that she is going to babysit our baby when I got back to work 6 months after the baby is born. We have already told her we are planning on making it work with our schedules and so on.

Well today she went out and bought baby clothes, toys, and a diaper bag! For herself! She said that she will need it all for when the baby comes over to stay. With her inability to take care of herself there is no way I am leaving my infant child with her alone! She even called my dads girlfriend to tell her that she will be doing all the babysitting. I am losing it!  How can I make her understand that she can come visit the baby but not watch the child unsupervised and that its OUR baby not hers?!?!?