Dogs are pack animals and they are behaving in a manner that has been instinctive for thousands and thousands of years. There is more to teaching your dogs to be separate than you might think.
It takes a lot of concentrated time and effort but you MUST do it or you will have two dogs that are out of control and never pay any attention to you at all when they are older.
They need to spend structured time away from each other, at regular intervals throughout the day. Start with five minutes and then slowly work up to one hour over the next two months. When they are separate is a good time to do some basic training (you know, sit, down, stay, come) and to teach the one that is on her own that its ok to be alone sometimes, to be quiet and calm and by herself. This is vital - particularly if you want to end up with dogs that have a good recall off leash, because together they will pay NO attention to you at a park or somewhere similar and will just run off together. This has happened with my dogs and it is scary, trust me!
Also, when the time comes for puppy kindy, don't take them together. They need to learn how to socialise with other dogs away from each other, and to pay attention to you in a situation where there are other dogs.
I mean, don't overdo it - they are sisters and will be great friends and a wonderful support for each other. But this is something that needs consistent effort on your behalf and you need to keep it up their whole lives, not just in the first few months.
When they get old enough to be taken on walks, too, for at least the first six months make sure that half of their walks are apart and half together. This is so that they can learn to behave apart and together, and that in both situations YOU are the one they look to and pay attention to.
You need to reinforce that when they are together, when they are separate, when they are around other dogs, they pay attention to YOU first and foremost, before anything else. There are some really excellent methods to teach this and I would be happy to help if you think this is something that you are willing to spend the time doing.
As for their fighting, don't stress about it. Again, natural littermate bevhaiour - they are just trying to sort out who's boss. Don't interefere and don't try and make that decision for them. They'll reach it a lot quicker on their own. It can get pretty full-on sometimes but don't intervene because you can make the situation worse and get yourself hurt in the process. Littermates play unbelievably rough, its pretty freaky. You'll know if they're really going to hurt each other - things change very quickly and get really aggressive with lightening speed. You need to be ready - supervise them when they tussle and if you think its going too far throw a blanket over them or squirt with water or a loud clap or something to distract them. But never, ever get in the middle and don't ever physically intervene. Bad idea and one that could get you hurt. Mostly though, I wouldn't worry - its a doggy thing and they do it to sort out their pack heirarchy and also for a bit of playful rough-housing. It'll get worse before it gets better!
Usually it is a bad idea to get related dogs of the same age and litter at the same time. However, its not a recipe for disaster as long as you are prepared to work a bit harder to reap the rewards later!