I think the whole point of this manga is to make the taboo of incest seems non-existence between the characters. That is why it lacks the tension of a taboo. I thought it is nicely done in the sense that it achieved its aim of making the taboo almost inconsequential, so will people stop gripping about that fact. Also, it is fiction, for goodness sake, not everything need to make absolute sense, go read non-fiction works of philosophers if you want something to make absolute logical sense. I feel that this manga is cute and interesting in its own way. I keep forgetting they are mother and daughter, the manga did a good job of minimising the feeling of incest here. Besides, if you think the drawing is bad and the characters look the same, clearly you haven't read a lot of comics because I encountered way worse many times, where characters don't even have differing hairstyles.