Even after living a happily married life with our respective partners, me and my elder sis can’t stay without fucking each other. Either she visits me when my wife’s gone or i go to her home
That’s the advantage of incestuous relationships. You can have your own partners and still be lovers for life without raising too much suspicion. Just create a reason to be together regularly.
And of course you already love and trust each other. So it’s just a small step for her to take, but certifies to make your lives much happier.
Go on girls, seduce your daddy (or your brother).