I don't know where to post this but I'm losing my mind and hoping someone has a solution.

So there are three dogs in this house: Zeplin (neutered male) is mine, and Duke (intact male) and Jessie (spayed female) are my sister's. Duke and Zeplin were the first two dogs to enter the house after I lost my furry best friend to old age three years ago. They bonded instantly and didn't have any problem adjusting. They ate together without fighting for food, etc. They were the perfect pair, honestly.

(Duke is the brown dog)

Then after a year of having them both, Jessie got dumped onto us. That's when Duke started to act aggressive towards Zeplin (and Duke is tiny compared to Zeplin so it was just bizarre). You couldn't have them both in the house without Duke instigating a fight. The fights were frequent and terrifying.

When Jessie was old enough to get spayed, Duke eventually calmed down. He'd still have the fur on his back stand on end when he would see Zeplin in the house with him, but that was it.

However, just the other day Duke was eating out of a bowl when something not-so-great happened. These dogs have shared food bowls before without a problem, like I mentioned earlier. But as soon as Zeplin got near, Duke went rabid and left a gash on my baby's face (picture was immediately after but it looked worse an hour later). These dogs have NEVER been food aggressive. NEVER.

And just now, I was trying to let Zeplin out this morning to go potty. This weird instinct told me to hold onto Zeplin's collar and I'm thankful for that because I didn't see Duke until after he started growling.

I'm getting frustrated because nothing is being done with Duke. He's not getting any training to chill the fuck out. He is the only dog in this house not neutered, and I think the fact that he's still intact is the reason why he's trying to fight my dog. And it's literally just towards Zeplin. He's a massive pussy around every other dog he interacts with. Or maybe it's the social order of these dogs. Duke was instantly beta when it was just him and Zeplin, but since the addition of the female it's like Duke is trying to prove he's alpha (lmfao he isn't though). It's this weird power struggle amongst these dogs. Duke and Zeplin have been with each other for a few months shy of three years, but you wouldn't guess that with the way they behave. It's driving me up the wall.

Does anyone have any suggestions (other than getting Duke's balls chopped off, which my sister doesn't have the money for)? Because I'm tired of walking on eggshells around that dog. I fear he's gonna start fighting with people soon (he's snapped at my nephew in the past btw).