Samantha might have had the body of a 13 year but thankfully her maturity was still in tact. So she decided to avoid whatever scheme Chelsea came up with. Just then a rush of new memories came in her head. Aside from the original ones of being the older cousin. She now had new memories of being the youngest. Being closer to Chelsea instead of Diana. Having playdates with her. Being babysat by Diana. Which Samantha found weird but given her current age.
Samantha just avoided Chelsea and decided to stick with the small talk with the other bridesmaids. Them asking how middle school was, if she had any boyfriends, etc. Which with the new memories she had, she was able to answer all of them. It was ackward knowing she used to be older but because of her wish, she was the youngest at the party.
Eventually...