So to make sure the readers know its not incest they made them step siblings on top of him not actually related to the father having double layer incest protection for the story.
I bet she only likes him cause when she was all sad and lost MC did not notice who she is and said something nice to her.
Thats all the backstory you need to know.
Its culture
I mean at a time in one area in the world a women who has a whale body type is considered 10/10 in most part of the world that would not have been the case today.
Thats just how their society works there is probably some hystorical reason for that like during that time fat=healthy...
The same author as that manga where the MC married his robot appliance.
Their art is good though dunno why this one is not great. If the publishing dates are correct the robot waifu one is his earlier work.