Well, hmmmm. My parents had children wandering in & out of their home on a regular basis along with their own 5 children. My mom insisted on house rules - if you couldn't follow them you weren't welcome.
On wm's second visit to my parents I remember my mom taking wm's face in her hand & stated very firmly "We don't do that here". Boy did wm clean up his act at grandma's house.
Having said that, your 3 y/o grandson to be needs to learn the rules of your home. If easy child & girlfriend want to leave R with you then they need to support the rules in your home. This will be a learning process.
