Stepsibling Rivalry
Unfortunately, your Stepkids + Your Biological Children = Disaster. The situation, while perhaps under control at this point, is not what you'd call amicable. It's more along the lines of “you stay in your corner of the world and I'll stay in mine,” which itself took plenty of time for the kids (who are now technically adults) to work out. In other words, as long as they don't have to interact with each other, everything's groovy between them.
Should They Be Invited?
Your daughter has now raised the valid point that she's having a smallish wedding and she doesn't want to exclude people she actually likes from the guest list just so her stepsiblings can be invited. Plus, she's sure the stepsiblings won't come anyway, so no harm done. You want to say something about this, but
For starters, if you're paying for the wedding and you want your stepkids included, they're in. That's one of the perks of being the host. Of course, if you're only paying for part of the gala (or your daughter and her beau are footing the entire bill themselves), negotiations will be necessary. Even though everything your daughter is saying may be right on the mark (you're fairly certain that the stepsiblings won't show up, and there is a very limited amount of space available in the reception facility), you should still try to convince her that inviting her stepbrother(s) and/or stepsister(s) is the right thing to do.
ALERT!
No matter what the relationship between the two camps, excluding members of the family from the wedding guest list is pretty harsh, and something that will be held against your daughter — and probably you, as well — for the rest of eternity.
Long-Term Effects
Excluding family members from weddings is the kind of thing that becomes family folklore. The story the stepsiblings will tell each other — and their friends and their other family members — goes something like this: “Janie is coldhearted. I always knew it, but not inviting us to her wedding just proved it beyond the shadow of a doubt.” This will be followed by other anecdotes exemplifying Janie's negative attributes and will be summed up by a group decision to never forgive her. Ever.
Offering the stepsibs the chance to attend (or to refuse to attend) the wedding takes the pressure off of you, at least. They may decide not to come, after all, in which case you and your daughter can both honestly say that you made the effort to include them.
