@Hello hello
Spoiler!
Liena warped everyone mind. She was the original wife not the current one. The current one was the sister. Hence why he literally can't sleep with our FL. Somewhere in his unconscious mind he knows that was his adopted sister and sleeping with her is where his limits are. He doesn't understand it at the moment either but he does love the FL. Just as a sister not wife. He loves the current sister given that was his original wife from what has been said by others.