I don't think you can book them in one transaction at the same time. In my experience, you will need to book them one at a time but you can use your
MDE for both. When I booked our 4 rooms, I was able to have 2 different MDE accounts logged in on 2 different devices which helped me to book them sort of simultaneously (as fast as I could click on them both, lol).
To book two rooms on the same MDE, you will need to quickly book one first and then book the other immediately after the first booking without logging out of your MDE.
There are things that you can do ahead of time to make it go faster like adding everyone to your MDE account (or linking if they have a separate account already). You can add your sister and her family to your account and make sure your account info is up to date with your credit card and address. This will save some time. I actually called Disney to make sure I had everyone added and linked correctly before FD came out just to make sure everything went smoothly.
I also recommend practicing booking before the promo comes out. Someone recommended that to me because 2015 FD was the first time I had booked online and it helped me be more confident. I just logged in and did a practice to make sure I wasn't giving the wrong information. You can practice going through check out as long as you don't hit that final submit button--you can cancel at anytime.
I also put in my dates before I went to sleep that night. Because the Disney site will often remember the last dates entered, I was ready to go in the middle of the night.
I hope that helps! Good luck!!!