Overly aggressive

OK so first off, we've had 3 cats all of which are brother and sisters. 2 girls 1 boy. The weird part about it is the boy did the thing thing with them and they had their own set of babies. One of the females is blonde hair colored and the other one is black.  OK so.. We recently got the male neutered and after we took him home, the blonde cat has started to fight him, and I mean all out fury. We had him run to my mother's room so he could be safe from being hurt after the operation but sometimes when we go to open the door to my mom's room, the blonde cat is there crying for us to open it and if he gets out she just goes crazy on him. My question to anyone here is why is she having such a horrible behavior with him? He's very passive so he just runs away. It's as if she's angry because he has nothing to breed with or something. :/

No, it's not because he can't breed with her.  It's the medicine he was given at the vet, the anesthesia.  It will wear off in a few days.  Just keep him isolated until then.  You need to anyway while he heals.  As for her, she needs to be spayed.  It's dangerous to keep her unsprayed.  She could develop pyometra.  Plus, if not, if she gets out and is in heat, she could keep having kittens.  It is very, very difficult to find good and responsible homes for kittens, let alone kittens over and over again.  Please make an appointment to get her spayed asap.

So because he can't breed with her and also due to the anesthesia, she's being aggressive towards him. Is it mostly because she senses the medicine or the fact that his stuff down there doesn't exist. 

It has nothing to do with him missing anything . In fact, if you don't have the females spayed he will probably still mount them when they're in heat. They don't forget how to do it. Keep in mind that he'll still be able to get them pregnant for a couple weeks because of sperm retained in the tubes.

He smells like the vet's office instead of himself, and is acting funny from the anesthetic. In a few days the smell should wear off and she should be OK then. If she's still not happy with him, try rubbing them with the same towel for a few days so they smell the same.

Interesting ;o. I will see how these days go by. Hopefully things calm down soon so he can be at ease but I thank you all very much hehe, I had a feeling this would be the best place to ask for this. I couldn't find an answer anywhere else