How would you handle this situation....not being invited....family....(read more inside)?

How would you handle this? I'm at my sister in laws house and my nephew asked me if I was going to a party at their house the next day and I say no. He asks why and I say I'm working (thing is I never even knew there was a party). Then he asks if I can get off of work and I say I can when I ask a couple of weeks ahead of time. He says well didn't you know? And I say, I don't have any kids and that's why I am not going (I am uming at this point that maybe my sister in law set something up so her friends and their kids can get together....and he says YOU HAVE ME! And it breaks my heart...then he asks if I was invited....and I tell him..well no but it's not a big deal....he shouts across the house to his mother. I try to get him to stop. Then my other sister in law who heard the whole thing chimes in that, Oh you aren't the only person she forgot to invite she forgot XYZ but that isn't completly true as I find out later.....