My dog gets aggressive to her brother

You have to tell her in her own language that her behavior is not accepted. Dogs do that by snapping (that is biting without actually biting). You as a human can do that by making a snapping gesture with your hand.

Here is a short video about it and famous "dog whisperer" Cesar Millan also explains it often in his shows, but I couldn't find a short video clip of it.

You already see the signs on an impending attack. You must react as fast as possible and "snap" at the side of her neck or torso with your hand before the attack starts. Tell her loudly and angrily "no!".

If she tries to displace the other dog when he gets attention, physically (but gently) push her away from you and the other dog. That tells her that she is not allowed to approach and chase the other dog away.

There is alot to dissect here, so here goes...

"my youngest started attacking my oldest when he was near her food"

This is perfectly normal behaviour for dogs, but food is the biggest escalator of problems and conflict. Feed them separately. Different rooms or areas of the house until they are better around each other. A sign of maturity now the youngest is getting older, they are becoming less tolerant and setting boundaries for the older one.

or when we give him more attention.

Again, normal behaviour, you need to give both equal amounts of attention or the other will get jealous. Equal attention and walk them separately for a while. If the other tries to wade in or attack/snap, a firm "NO!" should do it, if not send it away to its bed etc.

He doesn't retaliate

This is the last thing you want, a power struggle, retaliation will cause fights and issues, you need to stop it before it even gets to that. Making them comfortable around each other will help this.

She even takes his bones and he lets her.

This is where you need to step in, as soon as it takes the bone off the other one, walk over, remove the bone and a firm "NO!" and give it back to the other. If it persists then send it to its bed/crate.

Going near her would make her growl.

Again, normal, just being possessive of its bone, but follow the above if its taken it off the other.

I believe my youngest has self-esteem issues and wants to be loved, and we do love her. I give her first priority when i go outside (they stay outdoors) kissing her, hugging her all sorts of stuff, then my oldest. But she seems to want all of the attention. When i go to my oldest and give him attention my youngest goes near me.

I would say this behaviour is establishing it above the other in the pecking order, give both equal attention, not more or less.

A week ago i have my youngest half a treat, and was going to give her the rest, but gave it to my oldest. She became very aggressive and started biting him.

This needs to stop as it is causing friction and jealousy, both dogs are equal, you are the pack leader in the scenario. You need to treat them equally, no favourites.

Every time i have to hold her down very tightly or she'll slip out and start attacking him again.

Restricting may cause the dog to get more worked up and defensive as it is out of control of its own body, and it will feel vulnerable. Instead of distraining them, toys, food, play, attention, be fun, high pitch voices, you are more fun than what they are about to do this will focus their attention to you.

You need to get them comfortable around each other, this takes alot of time and persistence.

I grew up with dogs. As I'm the pack leader, any of my dogs attack the other dog, the attacking dog gets put on his back ("Alpha Roll"). This lets the dog know he's doing something wrong. The dog understands this, by the way, this is also something Cesar Millan does as well!

Please be aware this is not a first resort tool, use it if nothing else works. Roll the dog on his or her back, hold the dog down by placing one hand on the collar bone (you don't want to cut off the dog's oxygen supply). The dog will eventually yield by spreading his forelegs and rear legs out . When that happens, you let the dog back up. Cesar Millan uses this for aggressive behavior and it imitates what an alpha dog would do to a misbehaving member of his pack.