My Nephews birthday is coming up, but I curriently am not talking to his mom.

My "Nephew" (my brothers step son) is about to turn 10. But I am not talking to his mom (my sister-in-law) because of an argument we had over her calling my boyfriend a thief. I have been around this boy for 6+ years, and have literally helped raise him. But since his mother and I are not talking, how do I tell him "Happy Birthday". He is having a party soon, and his mother did not send an invite to me, I know he is going to be upset that I did not come, but im sure she would make a scein if i did. Do I call her cell phone and ask to talk to him? Do I just go over there when she is not home? Do I mail him a card, with his gift? Or do I just forget about it? Like I said... I have been around him for a LONG time and I know I have to do something! I love this boy as if he is my own, he didn't so anything wrong, and I don't want him to think I forgot about him.