Frankly, you don't really have to invite anyone. Of course, if someone is footing the bill for your wedding or a portion of it, they may expect that they'll have some say over the guest list. If you or your betrothed have children from a previous relationship, they really should be involved in the wedding in some fashion, unless they specifically ask not to be involved. And unless there are some particular reasons not to, it'd probably be essential to invite the immediate families of both participants. Otherwise, it's really up to you.