Welcome to Yorkie Talk and congratulations on the new addition to your family.
Sounds like he is guarding your mom and you because you are very important to him. Here is a brief article by trainer Victoria Stilwell. It focuses on food guarding, but there is some good information:
https://positively.com/dog-behavior/...urce-guarding/
My younger boy was 8 months old when we brought him home. Although he came from great circumstances, he loved his new home with us and I believe was afraid of being taken away.
Instilling confidence and a sense of security helps a lot. Have your brother interact with him in a very patient way. Don't rush things or force things. Positive training and rewards are key. Good luck!