So glad you provided this background. I do remember reading that backstory though I couldn’t find it with a quick look in my files. I know she hates liars and that is the conflict you have set up between them from the opening of the book. However, she is falling in love with him. She slept with him. He rescues her from the barn and gunfight. I guess what I am looking for is a major plot point in her character arc when she chooses to save his life and she gets in the SUV in the previous chapter. That is like forgiving him for lying. From then on she needs to be on his side, not angry, but worried and protective of him i.e. strengthen the love between them. That would make her strong and determined and remove the wishy-washy. That doesn’t mean she can’t be curious about her family and her mother.
Now if you want to continue with her hating him because he lied, then maybe you need to change the SUV moment and have her say something to Sergio like “Go ahead and shoot him. He’s a liar.” Then stomp off to the SUV. Then she maintains that anger at Eric all through the plane trip again removing the wish-washy.
In either case -instead of having Eric jump into the SUV after her, I suggest having Sergio tie him up and forcing Eric to go him with them under threat of death. That will add more conflict and tension as the reader will worry about what will happen to him. And Elise can be worried too.