They are his family and have been his family for 20 years. Don't know what society you're living in, but in the real world people can put up quite well with their family even without sex.
Having the hots for them is also as new for the MC as it is for his mom and sis. He catches himself looking at them in a way that makes himself uncomfortable quite a few times.
If anything it's all the other relationships, where the pacing is a little off. He goes from being at the verge of tears, because he kissed another girl, to being at work with his pants down, getting his dick fondled by his lesbian boss in the span of a day.
And only after the floodgates opened to being a cheating deviant, his long term girlfriend gets a proper introduction and turns into a likeable character.