Dilemma

[egooririexruinam]

Hi. I hope someone can help me.

I applied first to Uni A, then later at the very last minute to UNI B.

A professor from UNI A who interviewed me in December agreed to be my major advisor. He was so nice to me and was even the one who followed through with the committee about my application. He also updates me every now and then. I was eventually accepted and he was the first to let me know. I was delighted that I got an offer and my sister encouraged me to sign the letter and email it immediately as we were thinking that the other university would not make an offer anymore. So I emailed back the signed letter the very same day it was sent to me.

Exactly a week after, I got a very generous offer from a prestigious UNI B. We never expected that. Their offer was spot on and I'm really interested in their program because it's a better choice career-wise.

Now my sister wanted me to withdraw my application from UNI A and express my interest at UNI B asap. I don't know what to do. I'm very ashamed to email the professor at UNI A who was very nice and accomodating to every questions I had. I really want to join UNI B but I don't know if I should and how I should let UNI A know about it. 

Please help ???