Dogs can appear jealous but they aren't jealous. This is an emotion that is perculiar to humans despite how it may seem.

With dogs, it is purely down to conflict over resources and she is probably too used to getting sole attention when she wants it and, if she is with one person getting it and the other person approaches, she probably thinks it is threatened and so tries to defend it. You might think that this is jealousy but it is not. She doesn't care what the other person is getting, she only cares about what she is losing out on and she only acts like this when she has become too used to getting something and then it is taken away from her. A human can get jealous over something they never even had...this doesn't happen with dogs, they only get defensive over things they are used to getting.