I disagree. Your mother was saying to you, "When you leave your son in my care, I shall use my judgement about what is safe. If you don't think my judgement is sound, then don't leave your child with me." That's actually a fair statement, in my view.

I totally support your decision to not let your child be transported in a car being driven by someone who has been drinking. Though, come to think of it, that was not actually what you decided. When you saw the car in your driveway, with your step-dad at the wheel, you could have said to your mom by phone, "Come back immediately!" They couldn't have gotten far down the road when you had your mom on the cell phone. But that's not what you did. You decided that the risk to your son was not great enough for you to cancel your plans. So you let the car proceed down the road.

Ask yourself what it is that you are really angry about. I think you are more angry that your mom was non-compliant then you are about your son being put in jeopardy. This sounds like it may be something of a power struggle between you and your mom. (There's that word again.) Ask yourself if this is really only about your son's safety. Your mother doesn't think it is, and I suspect she may be on to something.

Your son's safety didn't require that his grandmother be ejected from the house. What was that really about? Are you using the issue of your son's welfare as a proxy for something else. Just think about it.

As time goes on, you are going to need help now and then with your son. You have no idea what situations may arise that will cause you to leave your son in someone else's care. You may find that there are no perfect babysitters out there and that you could do a lot worse than your mom. Outside of you and your husband, probably no one will love this child more than your mom. In would be in both your son's and your interests for you to cultivate cooperation from your mom in a constructive way, rather than by getting implacably confrontational.

I don't know your mom, and, for all I know, she may be someone who is not a good choice for a babysitter. That may be a conclusion that you will come to, and I would support you in your right to make that decision. I don't see it, however, as a reason to cut off contact with her. I think you are angry about something else.