Lying,Vindictive,Manipulative

My soon to be step daughter is 11. My fiance has 2 sons that are grown. The SD is a liar straight up! I've never seen a 11 year old little girl control a grown man. A few months ago she came to visit and had something on her lip. I looked at it because I'm a nurse and it's Herpes 1. So I tell my fiance she doesn't need to be kissing you especially on the mouth. ( I think it's inappropriate without the cold sore) So my fiance tells Bio mom to take her to the Dr. The next day he had her and she's kissing my fiance on the mouth again, and again I told her to stop. She then tells me that the Dr. said that it was a pimple. I told her regardless you don't need to be kissing your father on the mouth, cheek is fine.

2 to 3 weeks later my lip starts to tingle and I'm like wtf! I've never had a cold sore before. The next morning after the tingle I have this huge blister on my lip! Now I'm extremely pissed. My finance doesn't say a word to his daughter about Lying! Now we are 1 month from getting married and guess what popped up? Another cold sore, we are in La visiting his son and I asked him when are we going home? And the reason I asked is either I can hold off and use the abreva that I had at home or do I just need to buy some. He snaps on me yells at me when do you want to go home?!? During the entire time his daughter is smiling and laughing at me. I'm about to throw in the towel and walk away. Any advice