Trailer.

The guy in the white vest:

That was Thayer's violent & abusive older brother Dooley. (Dooley was wearing a white wife beater t-shirt in that scene).

Thayer's background & home life:

Thayer's Father constantly beat Thayer for minimal infractions...such as being called at home by Thayer's school principal...because Thayer had drawn a nude picture of Amber in study hall (to annoy her). The abusive Mr. Mangeress used Thayer as a punching bag & utilized Thayer's equally abusive older brother, Dooley, to assist him w/ Thayer's beatings. (Dooley would usually hold Thayer down, while Mr. M would whip him with his belt).
My personal opinion was that Dooley was an abusive schmuck who got off on Thayer's beatings.

Worse yet...Dooley would sexually abuse & rape Thayer's kid sister, "Jeannie". Thayer was especially protective of Jeannie, since he empathized w/her predicament. There was no one to protect Jeannie, if not for Thayer...just as there was no one to protect Thayer, period! Thayer & Jeannie were pretty much in the same boat. That's why Thayer understood Jeannie's position so well & why he was so protective of her. (There was no mom in the picture, either)


In answer to your question:

The scene you referred to.....
One night, Thayer is awakened by the sounds of Dooley moaning and grunting while Jeannie repeatedly pleas for Dooley to stop. From Thayer's perspective, you are now seeing into Jeannie's bedroom. Thayer sees Jeannie in her bed on her back, while Dooley is on top her. Dooley is trying to pull Jeannie's legs apart, while he attempts to rape her.

(Thayer, now a grown man, is almost as large as Dooley).

Thayer is angered & sickened by the scene between Dooley & Jeannie. Thayer grabs Dooley's t-shirt from behind, pulling Dooley off of Jeannie. As Thayer & Dooley roll around the bedroom & into the dining room, Thayer ends up throwing Dooley against the dining room wall. Dooley pulls out a knife and turns it back on Thayer, placing the blade against Thayer's throat. Dooley hopes Thayer will back down in fear, as he has before. Thayer surprises Dooley by grabbing the knife by the sharp blade, slicing his hand in the process...and yet Thayer never flinches. Thayer continues gripping the knife by the blade, as he pulls the knife away from his throat. Thayer eventually gains possession over the knife, beating Dooley in the fight. Thayer throws the knife down on the dining room table, as he yells for Jeannie to close her bedroom door & remain in her bedroom. Thayer ends the altercation by threatening Dooley... warning him to never come near or touch Jeannie again, or he'd end him.

The full movie is on You tube for free at:
(Sorry, you have to cut & paste the link, below)

https://www.youtube.com/watch?v=ru2D9JyRb_w

It is Titled: "Teenage Dirtbag - Full Movie"
And it is posted by the actual writer & director: Regina Crosby