(from left to right):my brother, my sister, my mom, and Me. Do you think my mom's ethnic background could be?

No one can tell anything from a photo. Order a copy of your Mom's birth certificate and find out who her father is. Contact the vital records department of the state where she was born. Each state has slightly different rules but as a direct relative you should be able to order it for about $15. You may need to wait until you are 18.