Back to the Future

Teenager Marty McFly is an aspiring musician dating girlfriend Jennifer Parker in Hill Valley, California. His father George is bullied by his supervisor, Biff Tannen, while his mother Lorraine is an overweight, depressed alcoholic. While dissatisfied with Marty's relationship with Jennifer, Lorraine recalls how she met George when her father hit him with a car.
On October 26, 1985, Marty meets his scientist friend, Dr. Emmett Brown, at a shopping mall parking lot. Doc unveils a time machine built from a modified DeLorean and powered by plutonium stolen from Libyan terrorists. Doc demonstrates the navigation system with the example date of November 5, 1955: the day he conceived the machine. A moment later, the Libyans arrive and kill him. Marty escapes in the DeLorean, but inadvertently activates the time machine, and arrives in 1955 without the required plutonium needed to return.
There, Marty encounters the teenage George, who is bullied by classmate Biff. After Marty saves George from an oncoming car and is knocked unconscious, he awakens to find himself tended by an infatuated Lorraine. Marty leaves and tracks down Doc's younger self to help him return to 1985. With no plutonium, Doc explains that the only power source capable of generating the necessary 1.21 gigawatts of electricity to power the time machine is a bolt of lightning. Marty shows Doc a flyer from the future that recounts a lightning strike at the town's courthouse the coming Saturday night. Doc instructs Marty to not leave his house or interact with anyone, as he could inadvertently change the course of history and alter the future; because of this, Doc refuses to heed warnings from Marty about his death in 1985. Marty realizes that he has prevented his parents from meeting and Doc warns Marty that he will be erased from existence if he doesn't find a way to introduce George to Lorraine. Doc formulates a plan to harness the power of the lightning while Marty sets about introducing his parents, but he antagonizes Biff and his gang in the process.
When Lorraine asks Marty to the upcoming school dance, Marty plans to have George "rescue" Lorraine from Marty's inappropriate advances. The plan goes awry when a drunken Biff attempts to force himself on Lorraine. George arrives to rescue her from Marty, but finds Biff instead. George knocks out Biff and Lorraine follows George to the dance floor, where they kiss and fall in love while Marty plays music with the band. Satisfied that he has secured his future existence, Marty leaves to meet Doc.
As the storm arrives, Marty returns to the clock tower and the lightning strikes on cue, sending Marty back to October 1985. He finds that Doc is not dead, as he had listened to Marty's warnings and worn a bullet-proof vest. Doc takes Marty home and departs to 2015.
Marty awakens the next morning to find his family changed: George is a self-confident, successful author, Lorraine is physically fit and happy, his brother David is a successful businessman, his sister Linda works in a boutique and has many "boyfriends" and Biff is now an obsequious auto valet. As Marty reunites with Jennifer, the DeLorean appears with Doc, dressed in a futuristic outfit, insisting they accompany him to 2015 to fix a problem with their future children. The trio get inside the DeLorean and disappear into the future.

Question: Who is Lorraine brother in this movie?
Answer: David; David