Send out an evite saying "space is limited, sorry, we can't accommodate siblings!" then call your friends who are exceptions to clarify. As a parent on the receiving end of invitations, I LOVE when it's obvious if it's just one kid or siblings are invited, I hate when I have to guess or feel it out. Not rude at all to be straightforward.

I don't know if Evite allows you to invite people by name, but Paperless Post does. You can say, "Due to space, we are not able to accommodate siblings."

It is a small party. Invite people by name. Write the names of both siblings that are invited. Put a note that due to the small space you can not accommodate any additional guests.

Send paper invitations, and address the invitation to each child you are inviting.

Larla Smith

or

Brayden and Jayden Jones

If a kid's name isn't on the invitation, he/she isn't invited. The real problem is that some parents will bring siblings who aren't invited. Saying "no siblings" seems rude, but it's only necessary because (rude) people presume to bring uninvited guests. They may be less likely to do this, however, if the invitations are addressed to specific kids.

Evite allows you to name kids (i think!) so, plug in Mrs. Smith's email and title it "Larla and Darla Smith"