Accepted in November 2019
That was marked wrong for me when i used it
Are you allowed to put multiple "de"s in a row like this?
Typically you would omit the first 的 to say: 她在她妹妹的旁边。
It seems the general concensus is to drop the first 的 like this for close relatives or people in Chinese. Ie, 我妈妈，我女朋友 but 我的鞋子 (my mother, girlfriend and shoes respectively)
Typically 的 is a possessive particle so like 他的苹果 or 她的妹妹