First of all, sad to hear about the loss of your brother. I have a brother, I love him so much.

Could there be some serious reason why your mom isn't telling you the name of your real father? Maybe he is not a good person, maybe he had done harm to your mom? How old was your mom when she had you? Young pregnant mothers are probably already quite scared about their whole situation. Maybe you mom's case was even worse since there wasn't a "stable" husband by her side for the much needed support. Maybe she REALLY does not remember, especially when that time of her life was so tough.

At this point and for the next hours nothing is going to change by not knowing your dad's name. If I were you I would try to calm down and see your mom's viewpoint. Of course what she did (keeping you in the dark) was not fair to you but I'm sure she had to do it to protect you.

For now calm down. Step by step you will get to the bottom of this. Good luck!