Are you saying you are unable to live a happy life because of these encounters? Are you a worse person as a result?

A person can have a lot of regrets in their lifetime, I don't see how this would outweigh any of the others.

Like I said, it depends on the perspective of the person in question. Assuming people won’t feel guilt after doing the deed with a sibling isn’t exactly the best mindset. But, it is more common for the person to not think anything of it simply because if you did feel guilty, why even do it in the first place? The outcome of your emotions is your own fault.