I would make it clear that each child and 1 adult can go for free. I don’t think you’d have to make it clear that siblings were not included, though.
In my experience, if a parent wants to bring a sibling, they usually check first with the parent organising the party, in which case, you could say, “you’re more than welcome to bring the guest’s sibling, but the venue is likely to charge entry, so you might want to check ahead if you’d need to book them for a general play”. The parents also don’t tend to expect party bags for the siblings, as they aren’t invited guests, but if you have some spare cake, sometimes it’s nice to offer the siblings a piece to take home.
Some parents do both turn up to a party with their child, which is why it might be worth letting parents know, when they rsvp “ah fantastic! So excited xxx can come. There is 1 adult allowed in for free for each child too, so nothing to pay, unless you want to bring your partner too, then there might be a charge”.
Also, completely not what you’re asking, but it’s always nice to have some snacks, or sandwiches for the parents at the party too. A lot of venues will do a plate of biscuits, sandwiches, or pizza for a few quid extra.