I totally understand the guilt. I have 2 boys and they are the light of my life so I don't know why 3 would be such a bad thing. But even though we love our boys, it doesn't change the desire for a daughter. For what it's worth, I know lots of people who have had opposite pregnancies but had the same gender, and lots that had similar pregnancies and had opposite genders. I don't think that neccesarily means much. The same goes for nub shots...I've seen and heard a lot of incorrect guesses. I mean, if they can incorrectly guess gender wrong at an ultrasound, then the nub shots have an even greater chance of being wrong.
I'm sorry for your struggles and your loss. I know it's difficult. Hopefully you will see girl next week at your scan and you can prove your sister wrong! If it does turn out to be a boy, I'm sure you'll think he's as amazing as your first son. I will say watching my boys be brothers is pretty awesome. Good luck next week!