No, you don't have to have someone from one account in all the parks (WDW and DL). You can definitely share with your DSis. As long as you enter your PP numbers within 30 days from the date your first pix was taken and then order the CD by the next 30 days (you will therefore have a total of 60 days possible on one CD order) it will work. I've done it countless times. Sometimes we have been at both WDW and DLR. Sometimes we haven't.
My Dsis and her DH will be taking their first trip to WDW in May. I will be sharing a PP CD with them. I'm sure DD11 and I will make several trips to DLR during the required time period.
Just remember, that as of now, the final pricing for your CD will be based on the last park you (or someone in your family) were in and had pix taken. So if you enter your WDW in your account and they you click on order the CD it will show $149.95. Once your sister completes her trip to DLR and then enters her card numbers, the price will then be $59.95. At one time the pricing was based on the last card you uploaded but Disney realized that was probably costing them money and changed it to the last park you are in.
When CD shares were allowed on the boards I had one member who found a card he forgot to upload (from WDW) until after I had uploaded my DLR ones. I was all ready to order and the price had changed to the WDW one! Yikes! I had to make a trip to DLR to get a few pix taken to reset the price. But that can't happen anymore as no matter when you enter your card numbers, your pictures will be displayed in chronological order.
Since you already prepaid for your WDW CD, here's what I would do...
Enter your WDW now and start editing. Since your sister is going next week there's no reason to hold off on your edits. She'll have plenty of time. Then after she gets back (or while she's there) she can enter her numbers into your account. I would then order your CD using the coupon code from the Photo Gallery CD. (It's also good for free shipping, $2.95). Once everything has gone through then go ahead and ask for a refund on your prepaid $99.95. That way, if for some unforeseen reason it doesn't work, worst case scenario is you will have only paid $99.95. This is exactly what I did the first time I ran a CD Share incorporating both WDW and DLR. I prepaid for my WDW CD and after I ordered it paying the DLR price, I asked for a refund. Worked like a charm! And that was when it was only $129.95 if you didn't prepay.
But this really does work, I swear.

I just ordered my last PP CD on March 17th and received it on Sat., the 26th. (took longer than I expected in the mail). I used my coupon from CD and didn't have to pay anything additional. I did have a friend on my CD who went to WDW.
Like I mentioned above, my DSis is going to WDW in May and she and I will share the PP CD, paying the DLR price. (Unless Disney makes a change to the way they have been doing it for the past 2-3 years, which I doubt.)
I hope your sister has a great trip.

Have her check out the Sticky thread on Photopass pictures people have posted on the DL thread. It'll give her an idea of some shots she can get. I have many posted there. Tell her to tell every photographer she sees that she is getting the CD and if they don't have a big line they will take more pictures. Have her ask them if they have any special ones they can take for your niece's birthday. We've done a few cute ones with DD when she turned 9, and 10. She held up the appropriate amount of fingers and we pointed at her, etc. DD11 and I are going to DLR tomorrow, Tuesday and possibly on Friday. Maybe we'll see them there.
Feel free to ask if you have any more questions, either here or you can send me a PM.
Hope you have a great time in WDW!