First, OP - if you're really worried, I'd just call CIL and tell her you're sending her an invitation although you know she can't make it, that you just wanted her to know you're thinking of her, etc. That gets the non-gift-grabby point across without risking hurting her feelings by excluding her. Or if you're not close enough to feel comfortable making that call, I would not invite her.
Second, to those who said a shower invitation means you ought to bring a gift only if you're attending, that's incorrect. A shower invitation customarily means you should send a gift either way. I assure you that people feel obligated to buy a present even if they can't attend. (Do you really get shower invitations and not send a small gift, just because you'll be on vacation that week or something? As in, "I can't come eat your food, so I'm not going to bother with a present"? I can't imagine doing that...)