So what if I'm insisting that my sister come to my housewarming party without her kid, even though she's told me that she can't find childcare?
Why would you tell her not to bring her kid?
I'm not interested in spending hours outside, socializing. I'd rather invite her over for geeky activities inside my home
You shouldn't invite people and expect to control who they bring. It's rude and very irresponsible. That's your nephew we are talking about.
Whatever, if she can't make it with her kid, she shouldn't come.
It's inconsiderate and hurtful to ask someone to not bring their child over when you invite them. You should consider what you would do if the shoe was on the other foot.