I'm at my wits end with this piece of work

You speak with your SO about how his mother openly provokes you and prevents you from parenting your step daughter. Tell him to have a talk with his mother and find solutions for his mother to move out of your house. Keep telling him that you cannot and will not continue to live with his mother under the same roof. The way he handles your requests will tell you what is your next step. Meanwhile prepare for the worst and speak with a lawyer about securing the custody of your son in case he decides to choose his mother over you. Hope for the best and prepare for the worst. Also, when you speak with your SO, make sure you have some recordings of his mother's behaviour to show him. To make everything clear, have the father decide who has the parenting role when it comes to his daughter: the stepmother or the grandmother (the parenting role cannot be split between stepmother and grandmother, the parent figure is either one or the other).

/r/JUSTNOMIL Thread