Hello, Lizbeth!
Welcome back! The answer to your question can be tricky, but I think you can make it work!
First, you can buy
Memory Maker before or after the trips. It is crucial that IF you plan to buy at the advanced purchase price, you do so at least three days BEFORE your cousin's trip to cover all days of the visit. Otherwise, there may be photos missing from the beginning of the trip. If you purchase afterward, then as long as you activate the product within the photo expiration window, you should be okay. Disney PhotoPass Photos expire 45 days from the date taken.
Second, you need to decide who will hold the Memory Maker entitlement. Only one person can download all the PhotoPass media (photos and videos). This is the tricky part. Therefore, if your cousin wants to share photos during her trip and you hold the entitlement, then you will have to download and send each one to her. If she holds the entitlement, then she will have to do the same for you during your trip. Now, if you do not activate until after both trips, then you can be the entitlement holder and share the photos with your cousin. Remember that once activated, you have unlimited downloads all of the media in your account for 30 days.
Finally, for any of this to work, everyone must be
linked in Friends & Family via My Disney Experience accounts before the visits. You also need to be sure that everyone gives permission to the entitlement holder to view AND purchase photos. This is essential to ensure that all the photos show up in the account and are part of the Memory Maker download.
I hope this information helps! Please let me know if you have additional questions!
Missy