what to do when the baby comes

Danielle
Here's a little background so you understand. 
My SO is overseas so I moved in with my mom and little brother for help during my pregnancy and first 6 months of my babies life (SO will be back in May and I'm due in three weeks) I'm really happy here, my mom helps me out and I pay some bills but she won't let me pay rent so I buy the groceries and pay for extras like take out, cable bill and cell phones for us 3.. It's working out pretty well. 
The problem is my MIL. She really two faced and fake. She promises so much but does nothing, talks about me and my mom all the time and it always gets back to us. She currently dating my cousin (who is a loser and alcoholic, doesn't work just stays home all day drinking beer and smoking weed) She hasn't seen me since I told the family I was pregnant (I was maybe 12-14 weeks at that time) even though she lives 5 minutes away.. She talks a lot about how she's going to be the best grandmother but I honestly am not comfortable bringing my son around my cousin (who she lives with) for one, the drinking is my main concern.. He's not an anger drunk but tends to fall over and act stupid.. Secondly she hunks she knows everything about kids (her youngest is 20 smh) and I fear she won't obey my parenting rules.. Also my mom says she isn't welcome in her house (which I completely understand given what she has said about her and me) 
My SO is very laid back and allows me to make the desicions for our son but I don't know how to go about it.. I don't want to completely ban her from seeing him but how could it work? 
Any advice would be appreciated.. Really not sure how to go about this.