did your family try talking to her already.  i think your sister should be allowed to air her side of the story.  she must feel the way she feels for a reason.

Oh! your sister has a   feeling of insecurity, and why she feels that way? She maybe needs an attention from your parents and from the other members of your family. She needs your love and understanding, give those to her.

Don't let your sister's insecurities be grow into  hatred which is intended to all of you. Her feeling is based of the experience she has felt of  the way she has been treated by all the members of your family.

Be a friend to your sister. Let your sister feels that she is loved and cared for.

If  she feels that she is being loved and cared for , her jealousy is eradicated. And it would be replaced  by being confident

Some parents have been treated their children in a way of  showing favoritism, they may not mind it but the unfavored child feels the unwanted, may rebel, to get their parents attention that she is existing.

I understand your sister coz Im a middle child too and it can really seem as if all the attention is on the other siblings.  But my mom talked to me and set everything right.  Although I didnt get it right away, the talk helped me realize that they really do care and will open up if you talk to them.

I always tell my parents not to freak out on me.  This way we all avoid all the unnescessary drama.  I tell them mano a mano and they talk to me right back. Having uncomplicated lines for communications can go a long way.  This is what they always tell me.

give some time to explain everything to her, after all, you're her big sister. she may not understand it, so make sure you make her feel important and loved. that's the least thing you can do for now.

since you are older than her, you can give her advice. do it in a nice way where she can better understand.

you can always talk to her, i'm sure she will listen.