Why is my dog being aggressive? What can I do?

[Carolina M]

My female great dane has been aggressive towards my sisters dog who is also a female and attacked her but shes ok, it seems that she started being aggressive towards the end of her heat cycle which is her first heat and she got it at 14 months, which is what I think has caused the aggression, I have not bred her so it cant be that shes pregnant, and we have another dog but she doesn't do anything to him, and apart from that he is a submissive dog, and my sister dog is not. For the bleeding part of her heat I kept her in my laundry room to sleep and only took her out to use the bathroom, but the thing is that prier to her attacking my sisters dog they had even slept in the laundry room together and didn't do anything to her, and then like 2 days later she attacked her, and every time shes sees her shes always trying to be submissive, and growling at her, but she doesn't do anything because me and my sister are always there when they're together, but you can tell that she wants to do something to her by the way she stands over her. My sisters dog is a small pit mix and I'm scared that my dog might do something serious to her since shes like 4x bigger then her, my dog has been socialized with other dogs since she was a puppy and has never attacked another dog, ever, she knows basic commands, and she knows that I am the dominant one, I have always been the dominant one with her. is it possible that its because she has reached sexual maturity, and if it is will it pass, or do I have to do some training or what? and yes I know I have to spay her and I will I just want the heat to pass completely because I know they will charge me more, they did that to my mom whos dog had been out of heat for 2 weeks and still charged her more. And the aggression is only towards my sisters dog, not people cause alot of people come through my house, and I haven't taken her to the dog park since she had been in heat these past weeks.
I know that's what I was telling my sister, but my sisters dog is not submissive in the slightest, and my sisters dog will fight back, and if that happens I know something really bad is going to happen to the point where my dog can kill her, and I don't want that to happen.