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 ... 

Glow Resources

Let’s Glow

Glow is here for you on your path to pregnancy

Glow helps you navigate your fertility journey with smart tools, personalized insights, and guidance from medical experts who understand what matters most.

25+ million

Users

4.8 stars

200k+ app ratings

20+

Medical advisors