Am i wrong

My sons dad is 29 will be 30 in a few months. He has decided to take his little 17 year old cousin's gf that is 18. What he does is him an i dont really care much. But for starters he rarely get his son maybe a few hours a month. I have never told him that he can not get him, but bc of his drug use an company he hangs out with i have put rule on him getting him. Like where they go an what they do. He never just gets him an takes him to park he alwayz want to get him when he is with his friends or whatever girl he is with at the time is with him. I have never said anything about the girls bc they was all around our age with kids of their own it made me feel better like they would look aftwr my son if he wasnt but now that he is with a 18 year old that lives with him in mis mother house i dont want my son thinking that this is ok. So i have told him that i dont want her around my son. I know how i was when i was that age ..i told him let me meet her see what type of,person she is an if you are forreal going to be with this one i will think about my son,being around her. But he says no im stupid...both of them sittin around all day an smoke weed she dont work an,he works when ever he can find a day job for that day.or something that only last a week or two..(btw i dont think there is nothing wrong with smoking as long an you have your shit prioritized).thoughts?