Hmm, how about...

Thankyou for the invite, but we have some things going on this weekend and :Insert Sons Name Here: is studying for the SAT's and I need to be around for him to ask me questions. I really wish I could come to ::insert neices name here:: party, but it just doesn't seem possible. Please give her a hug and kiss from me!

LOL, I dunno. But then you aren't lieing. You DO need to be around for him to ask you questions!

Ash