@nobaka
They explained it earlier on, most demon familys are like that. Their child (and wife to an extent) can be very dangerous, even deadly, so fathers usually leave for their own, and their family's safety. Most demons do not (or at least did not) trust other demons. Often times it would lead to a fight where one would possibly kill the other. So it was a "rule" to basically live alone, making families a very rare thing. It is also why the demon population is so small when compared to humans.
In Lorem's case specifically, she is considered one of, if not the most, dangerous. Only 1 other being in existance can not be harmed by her heat, and that is Gospel. Even her sister is not safe from it. It is just Lorem's protective nature towards those she cares about which keeps her sister safe.
Look at ch. 20 or so. Lorem used to get challenged to fights a lot. She was dangerous, so most wanted her dead, even if only for the peace of mind. But they could not kill her. Her sister was also sought for, since she can heal just about any wound, but Lorem would always protect her.
But Lorem is not the smartest, nor the most careful. She left an anti-demon bomb in her house as a souvenir and all. So the father not being there is almost certainly (if not only) for his own safety.