I know some people can't take this anime due to its incest element shown in the series however, like what one of the character said in the end, it may be wrong and hard for the two to be together since it's morally wrong to the world but then again, you can't help falling in love with someone when you do, it just is....We as the viewers have to acknowledge the fact that this situation exist around the world and who are we to stop it? I personally believe that when each person is born in this earth, whether you're straight, gay, siblings, bisexual, trans...etc, whatever we categorise and call ourselves these days, fate ultimately choose your partner/lover in the end. There is no rules to how you should love, it just is and is fate whether you end up marrying that person or not, divorcing them, leaving them, staying with them, dying with them, it will counts on destiny. Only people around you can influence your decision whether you like it or not, that why it's always best to let your heart guide you to wherever you need to go.
I think the love between the brother and sister in the series tells us a great fact that we are all humans, and even though we have morals to stand and live by, when certain things end up the way it is, it doesn't mean its wrong, it just is. And I think that's how we all stay true to ourselves.