A friend of mine has a daughter who is about to start kindergarten who has been experiencing some anxiety about it. The anxiety has resulted in tantrums when the routine isn't followed at home. She asked for my "teacherly" advice, and here was my response:
School will be a big adjustment initially (has she been to Pre-K?), but there will definitely be a routine in place there which should help her out a lot! Especially if you establish a routine for what happens when she comes home from school (i.e. snack, homework if any, playtime, supper, bath, story, bed). I would not hesitate to talk to the teacher about her anxiety in regards to routine and make him/her aware in advance. Teachers always love when you tell them that YOU know your child isn't perfect and you are working on things at home. Home support goes a long way when your child has any issues, or the potential to have them. You guys and the teacher will work as a team to help her make the kindergarten transition in a positive way.
Now is a good time to pick up some books about Kindergarten, and I'm sure you've been talking to her about going to school, etc. Some of the best ones I know are:
Miss Bindergarten Gets Ready for Kindergarten (Slate & Wolff)
**There are a ton of books in the Miss Bindergarten series that are so cute!!!
The Night Before Kindergarten (Natasha Wing)
Look Out, Kindergarten! Here I Come! (Carlson)
Regarding routine at home, have you tried making a picture chart for her? Some of our younger grades use these at school, and in special ed classes, each individual student has their own routine chart with pics. (I'm not suggesting your child is special ed!). Anyway, the idea is that you chart out what the routine is each day and it's represented by pictures so the child can follow it. You might put times on it to make it more specific. The ones at our school have velcro spots next to each "task" in the routine, so the child puts a reattachable object next to it when it is completed (or you could laminate it and let her check off each task with a dry erase marker). I'm not explaining this well, so check this website out:
The picture cards are here in clipart form, but you can also maybe even go through magazines or old books with her and let her choose the pictures for each task, making it into a project that you guys work on together, allowing her more ownership:
Emphasize that this is what "Big Girls" or "Kindergarten girls" do...they can make right choices by doing what is asked of them, and follow adult directions...the first time. Working on that skill alone (following adult directions when given), will make her more successful in kindergarten than you know!
You may even want to do a secondary chart with stickers...where if she follows the directions/routine all day one day without a meltdown, she gets to put a sticker on the sticker chart. When she achieves say, 3 stickers in a row, give her a SMALL reward. Maybe after a straight week of stickers, she gets a little bigger reward. When she fills up the sticker chart, then I would introduce the bunny...and add a task of "Feeding bunny" or "Cleaning Bunny Cage" to her task chart. Kids feel so grownup when they have a job to do, and pets are great way to introduce that responsibility.
In regards to rewards: be wary of food rewards. Think of things like "computer time with Dad" or "playing outside with Mom" or "a trip to the library" or a big one could be "choosing a new book/toy at Target." Don't let her assume it will always be a piece of candy, or necessarily an object.
Finally, both of your girls are really old enough for the routine charts. You might start the younger one on a modified version, but if they both have it and Elena's has a few extra steps, she'll feel more special. Be sure to place the charts somewhere like the kitchen where the whole family can see them all day long and chart the progress being made.
Those are just some ideas from my end. I hope they help!