As much as I believe horses feel emotions, what we would describe as jealousy isn't one of them. This is a classic herd bound situation. He is her leader, and it sounds like he hasn't hesitated to be aggressive about it to show her, he may just have a larger personal bubble that she got in the way of a few times. This may be why she is nervous around him. If he's still relatively new, she's still learning his personality, how close is too close, etc and she's just being cautious.
But these two do need each other. Is it just the two of them together? That will make the bond even stronger. If you can, try riding them in an area where they have a visual on the other while you work on this issue.
Take it slow, every week halter and lead one away from the other and stand for ten to 15 minutes, praising when they are calm. Work on easy ground cues to get their focus on you. First week, only go ten feet from the fence where the other horse is, do this until they don't mind, then go twenty feet and so on. It's a long pro leadership in these situations you will have overall calmer, happier horses :)
Posted via Mobile Device