They also made fake profiles for the student on dating websites, she said, where they left her cell phone number and said she would "give herself for sex for free".
On Tuesday, Jacqueline was at home when she heard her sister crying in her bedroom. The tormented teenager also sent her siblings a text message to say goodbye.
It read: "I love you so much just remember that please and I'm so sorry for everything."
She died in hospital on Tuesday hours after she had tried to kill herself.
Ms Vela revealed her sister had always been bullied for her weight, but in recent months it had turned into a relentless campaign of cyberbullying.
She told CNN: "People would make up fake Facebook accounts and they would message her and she wouldn't respond and they would still come at her.
"They would say really, really mean things like, 'Why are you still here?' They would call her fat and ugly. She was beautiful, absolutely beautiful; the only thing people could find to pick on her was her weight."
Jacqueline ran towards her sister's bedroom where she found her standing against the wall with the gun pointed at her chest.
She pleaded with her not to shoot before running to get her parents for help.
The 22-year-old remained in their room while they, alongside her grandparents, tried in vain to calm Brandy down.
"I was in my parents' room and I just heard the shot and my dad just yelled, 'Help me, help me, help me,'" Jacqueline recalled through tears.
Neither of the girls' parents spoke of their daughter's death on Wednesday.
The family claim they reported bullies to police several times before Brandy's death but nothing was done.
Victor Vela, Brandy's brother, issued a sinister message to her tormentors. "I'm glad you got what you wanted. I hope this makes you happy," he said.
The school district said coming to class was a 'safe environment' for the student.
"School was a safe environment for Brandy," Melissa Tortorici,Texas City Independent school District's director of communications, said in a statement.
"She had a lot of friends and was thought of warmly by her peers and teachers. She did bring it to the school's attention before Thanksgiving break that she was getting harassing messages to her cellphone outside of school."
Officials could not pin down the bullies at the time and told Brandy to change her phone number instead.
"Our deputy investigated it and the app that was being used to send the messages was untraceable. We encouraged her to change her phone number."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
