I'm not that anon, but basically he's right and you're stupid with no experience. I've had this argument a million times with the permavirgins of /a/.
Your cheesy excuse of 'most of my family is with their first romantic partner' implies that you know nothing about the reality of relationships. I'll spoonfeed you here:
THEY SETTLED. Most people in long term relationships settle, not stay in them because they are actually happy. It's estimated less than 20% of long term (10+ yrs) relationships are actually happy and functional. They settle due to lack of confidence in finding a new partner due to age, financial responsibilities, children, etc etc.
The other anon is right, just like with co-workers you don't shit where you eat.
3D is not 2D. Stop trying to mix your stupid fantasy into reality. Fucking your sis might be hot if she wanted it bad, but you would both regret it 10 years down the line. A shitty ex you can leave in the dust and never see again in your life, your sister is your sis forever.
There is no argument, common sense, experience, and statistics prove you wrong on all fronts. Enjoy your fantasy, and keep it that.