Hi Nikki!
Thank you for bringing your question to the Disney Parks Moms Panel!
I was in the same boat last year. I had to register my husband, my son, my son's friend and myself! I knew it would sell out quickly, so I was logged into Active.com ten minutes before registration opened. I had no idea it would sell out as fast as it did, or I would have had my husband doing some of them! Luckily, I got all four of us signed up.
To answer your question, you can only sign up for one person at a time. If you have their information, you can sign them up. But, I recommend dividing and conquering. If you and your sister-in-law are both registering at the same time, it will increase your chances of getting everyone in.
It may be that the race doesn't sell out as fast this year (they hadn't been selling out nearly that fast in past years), but you do want to be prepared. Another option is to go with a Travel Provider for registration. You can find information about this on the
Wine and Dine website under the tabs "REGISTRATION" and then "Travel Providers".
I hope this helps! Please let me know if you have any other questions.