she sounds just like my niece and nephew.....they are absolute little ****s for my sister in law and brother and are angels for everyone else. It is all to do with boundaries and consistent follow through. She is good for others because she doesnt know how far that piece of string will stretch. She knows that she can stretch it further with you and Jacob and so will continue to test that boundary. Some things we were taught in my degree in child psychologist (though I am no expert...have no kids yet so these are just ideas)...make sure you are at their level, maintain eye contact be frim keep voice low in tone and most importantly agree with jacob on what a consistent follow through action will be...maybe if she cannot yet get out of her cot put her in there for a few minutes with no stimulus. one key is that the punishment (hate that word but cant think of the one I want) is that it should be less agreeable than doing the right thing so removing any stimulus is a big key...lots of people put kids in their room and shut the door for some peace, forgetting that the room is full of exciting toys to play with. It is hard with her age though she is not quite old enough to reason with effectively and too young to do things like the "naughty chair" etc. Good luck
Mel