So it pretty much says it all in the title. And  i feel it for two years already, So it's not just a whim.
But my girlfriend is a good person, pretty,   overall a good catch so i don't wanna just break up with her for the slim chance of getting the sister, but if we stay together,  i just think i will have this feeling for my whole life that there's so little separating me from something so much better. To make a sports metaphor, it's like if you compete at the Olympics and finish second - it's still a great result, but you didn't win. And that's how i feel
Also the issue might be the age difference, I'm 30, sister is 20

Please don't write the negative stuff, i googled this issue already and mostly it was negative comments, so focus on the possibilities instead