The father clearly raised his daughter - Dad is the main thing. You could always find support and encouragement from him. And to suck his cock is just a thank-you for having him. By pulling her on his cock, her father showed how much he trusts her and that secret will be with them now. And the chick did a great job - and daddy is happy and she's even closer to him now.
Her sister got hysterical over her boyfriend who drew a slutty picture of her - how skinny and flat she was. Her brother calmed her down and measured her waist and hip size, assuring her that she was awesome! Sure, her gratitude was inadequate - sucking her brother's cock, but didn't the girl deserve sympathy? When she wanted to remove her head already, he wouldn't let her - if she wanted to be a grown-up, then swallow it. And it seemed his sperm was to her liking. Now she could always count on him.
That's awesome.