Birthday party dilemma

Ok so my sister is throwing a last min birthday party for my nephew turning 17. Really last min .
My SOs nephew had asked to stay over and wanted to go to the party . I had told my SO no because 1) it's last min party limited portions 2) it's kinda rude to just show up with an extra random person. 
Am I wrong to saying not to bring him?? 
Hate being the bad guy but the two boys have only met one other time ...