We're sending our STD and invites to everyone. There are a lot of family members that we know won't be able to come (for a variety of reasons) but we don't want them to feel excluded. You also never know who may say yes, and see this as a reason for a vacation themselves. I think the money spent on the invites, postage, etc is worth not making your family members worthy of being invited to your wedding, even if you know they won't be able to come. At least that's how I would feel in that situation.