Just a remark about the first thing, you are aware that incest has been practiced by humanity for as long as it has been around and was in the past even the rule in some families, since they thought their bloodline was sacred and should not be sullied by unworthy bloodlines? Thing is however you may think about it, there are situations that can make incest pretty "normal" to people just as you do not think it is normal.
That those things most of the time will have to do with things happening in youth that your average psychologist will not consider as a healthy upbringing/emotional state or will have to do with not much choice of other viable partners being around is another thing.
Joke is that those reasons are hardly ever used in these games though and it is mostly just MC is horny and has irresistible big dick, while MC can get plenty of pussy and the mom and sis look like women that can get hundreds of guys if walk into a bar and show interest in sex, while have no good reason to be interested in their son/brother in the first place besides the magic dick.
So yeah most of the time it makes no sense at all they end up together.
Now the thing with this game is as Dev's said Daisy comes from an incestuous background and she does not see it as a bad thing, but as a good thing. She loves Mc and wants that good thing also for him and for herself to be able to continue her incestuous life style. So she will try to put things in motion to accomplish her goals and she will be a part of that or want to be.
Now another thing is if she is a bit too hands on or predatory/manipulative and does not show enough care/love to MC as well in that she can become unsympathetic. That is what devs need to guard against, since as a player we will need to root for her to succeed in her plans. Considering her plans and intentions though you cannot make it completely avoidable, seeing her with others, she is fully planning to be a part of that incest harem with her love after all and share in his incestuous life.
