I am stressed because of loving the sister of my fiancée.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The scholars have defined and discussed in details love and his ruling in Fatwa 84544. Nonetheless, it should be noted here that whoever is afflicted by the love of the sister of his fiancée, then there is no harm on him to nullify his engagement with his fiancée and ask the one he loves for marriage. He should do so with wisdom and appropriate means. However, if he thinks that by marrying his fiancée he would turn away from her sister and he would not be attached to her, then he has to take the initiative to marry at the earliest. 

Allaah Knows best.