I hate that term "dominant" i see it more as resource holding potential. However, my 14 week old pup seems o have a bit of a dominance obsession over other dogs. Shes absolutely fine with people, not pushy in any way. However  when she meets certain puppies she wants to assert herself over them. She's fine with adult dogs, although is a typical confident slightly pushy pup, but she accepts being put in her place which is fine. However after she first left the litter, she met her sister again after a few days away, she growled and tried to pin the other pup. After a bit of posturing they were fine angain and have been ever since as they spend a lot of time together as she lives with my boyfriend and his parents. At puppy training the other day she met her other sister whom she hasn't seen since 9 weeks and she was positively vile to her. My pup was always the hooligan (more confident) one and the other was the most submissive wriggly one. Every time they met on lead (there was no off lead its all controlled) she gumbled and growled and tried to jump on top of honey. It didn't seem to bother the other pup that much, but it sounded quite nasty from molly and i don't think she is going to let it drop. You'd think she'd be please to see her?!
Finally a friend at work has a slightly older jack russell, a lovely little chap who isn't terribly confident with other dogs. They've met before but the last couple of times they've met Molly has to go through the growling and the posturing for a few minutes before she will finally decide to play nicely. Other pups she will play with no problem right from the start.  Its almost as if she is picking out the weaker ones and trying to see how much she can push them. At the moment i'm trying to avoid it by taking her away when she starts, but i'm not sure how to stop her doing it in the first place or why she's even doing it. She's going to get herself into trouble if she continues and irts not nice!

No ideas?

I don't know enough about puppies to give you any advice, but it sounds to me like your pup isn't being dominant, she's just being a pup.

When I got my pup I had an adult male allready. When they played it sounded as if they were going to kill eachother but it was only play. Pup would trot up to adult looking all cocky then pounce on him, it was his was of starting a game. If the adult wanted to play they would have a right ol' rough & tumble grabbing each others cheeks/legs/necks, jumping on each other while growling.

I was always told to not stop them playing, as the adult would teach the pup the 'rules', but I found I had to step in as pup would get too rough, if my adult dog wasn't an oldie I would have left them too it as there was no aggression there, just a pup learning how things are done through play.

Hopefully somebody with experience can give you some advie.

It does sound as if your girl is a bossy mare and with her siblings ruled the roost so when meeting up again is just telling them that she is still head girl...lol
I wouldn't worry too much with pups of her litter as pups do sound nasty sometimes to each other. The best way to cope with her attitude is to ignore the bad and praise the good when she does her greeting even if it is bossy call her and give her a command to sit for you, get a favourite toy to distract her and give a treat she will soon learn how you [her mum] likes her to behave without shouting and jumping into to settle any squabbles for her. Its all a big learning process she has to go through on her own.
Working on the basics- sit,down, leave, stay, etc- even if she is very confident and forward with other dogs she will ultimately listen to you as she gets older. She is still only a baby so fun and games to bring out her sweet side with other dogs around. Its amazing how quickly a dog can pick up our anxiety about other dogs being around so leading them to act over the top in trying to control the situation. 

Dogs are pack animals and hierarchical to boot. We just have to accept that a group of dogs together will at some point test each other and suss out who is weaker and who is stronger, though this is fluid and can vary from situation to situation. Older dogs tend to do this in more subtle ways because most do not want real confrontation and so go through a series of postures and bluffs to see who blinks first. Pups do all of this too but are much less subtle because they are still learning doggie body language. Some pups are just bigger personalities than others and more inclined to push the boundaries. 

As Magica says it does sound as though Molly is a more confident and bossy pup. On the plus side she'll probably be a quick learner and may end up the kind of matriarch that rules the roost. Pups learn from each other and from adult dogs about dog language and rules and their place in the pack. This is only natural and normal.

The only thing I would be a little careful about is allowing a much bigger breed puppy to bully a very underconfident pup of a smaller breed. If you feel this is happening don't make a bit deal of it, just avoid the situation or stop interaction and move on to something else. Again, pups can bully, it's fun for them, it's up to us to step in as necessary.