Correct etiquette would be to send each sister an individual invitation, regardless of whether they have a partner or not. If you are planning on inviting each of them AND a partner for each, then they should definitely be receiving individual invitations.
According to the proper etiquette, the only time it's appropriate to include other people on an invitation within the same household (if they are not a couple) is if the guest is a child. BUT... if you want to save on an invitation, and you don't think they would mind, then just use one. If it were me, I'd send them both an individual invitation, but I am a real stickler for etiquette.