I think the first step to being supportive of your sister through your jealousy is to remember that it's okay for you to feel jealous. Uncomfortable feelings aren't bad, feeling them doesn't make you bad. Be in it, allow yourself space to feel jealous.
Get a notebook and fill a page with your disappointment. Doing this means you're handling the jealousy by facing it and acknowledging it without projecting it onto your sister. You're transferring that jealousy in a healthy way. Make a routine out of it.
This way you have an outlet and when your sister shares her joy with you, you can take part in it without dumping your negative feelings on her. Separate the joy from the jealousy and feel it all, in the right places and at the right time.