Yep, if you want to leave the world of manga and talk serious, romance between former family members is bound to be emotionally screwed up, and not worth defending hard. Then again, that also applies to... relationships in general. People are selfish and immature, and mostly pursue relationships to fulfill some shallow emotional/sexual needs.
The world of 2D is all about ideals. In this manga, the love between the characters isn't due to some emotional manipulation, but rather a result of their deep relationship naturally evolving in a romantic direction. Hooray for wincest!