OMG!!! I didn’t think of that! Omg I love that plot twist.
I would just have to tell the uncle lies
╮( ̄▽ ̄)╭ , I would be like “ Uncle you heard the rumors, he is a monster, why would you give your precious daughter to a monster. He might kill her. I love my cousin and am willing to sacrifice myself. Right now she might even be under a spell and is saying she wants to go and then get tortured by that monster. Locked her up until I go to the Halstead castle, don’t release her until the marriage ceremony. I want the best for my cousin and I know you do too.” I hope that would work and if she gets released and goes to the castle it will be too late. I’d be married to him already!

Gets reincarnated as Leticia:
Me: Send me instead! Uncle I will make the sacrifice! I will go to lord Halstead house and become his bride.
In the carriage to go to become the bride of Erden.
Me: fu fu fu fu fu fu I am coming future sexy husband! (/•ㅂ•)/✧ Wait for me!