I've been invited to destination weddings and never had he venue paid for by the host, so there shouldn't be an expectation on anyone's part that it's paid for. That said, if she wants to offer to pay for close family or some other group, I don't see that as a problem. I also wouldn't send out an email letting those who are not paid for know others are, since it is bound to cause problems.
In general, if you're having a destination wedding and want relative X to come, who you know has tight finances, you may need to either pay for them or accept that they cannot afford to come. For close family, I think some sensitivity is in order, so if you know that they are unlikely to be able to afford to come, choosing a close venue or helping them afford it is probably a good idea.