Hello, I have a 7mths old lab bitch and have the chance to buy her sister. I have heard that while they are young they will be fine but as they get older there is a very good chance that they will start to fight due to a domineering situation. if this the case or just a small possibility??? please help as i need to make the choice soon !!

I wouldn't do it there are no advantages that I can think of and possibly quite a few disadvantages.  You really need to be able to concentrate on yoru girl until she is at least 18 months old, she hasn't even hit the teenage stage yet. :D

Get another one when she is adult and her training is as you would wish it.