Help!!! “Slept with elder sister but the younger sister is my fiancée”

Need advice because am hopelessly in love with my fiancée; I am 30 years old and my fiancée who also loves me so much is 24.
When I first met her, her elder sister seduced me and we had s*x. Though I regretted this, I have made my intentions known to their family members that I want to marry the younger sister. But the trouble is that the older sister has vowed that.

She would not live to see me marry her younger sister and even told everyone that we had s*x together. I have tried severally to make peace with her but she didn’t agree. Now, I can’t face my fiancée to tell her the truth that I slept with her own elder sister.

What do I do? We are so much in love with each other.