Sarah while absolutely being a young teen, isn't wrong here. When she talked to Jordan she asked him for why. Ie a valid reason for his odd behavior, his lies, his running off, missing dates. He couldn't provide one, And in the real world that's a huge red flag.
Well she is now told part of that issue (merging world, and he has powers, she doesn't know all. But that is enough and a valid reason. Meaning if she was told some of that previously, She would have trusted that enough to stay with him.
I certainly don't blame Sarah for that (some other things sure). But in reality I blame Jordan for being to dim to come up with a lie that's close enough to the truth but not the truth to get the same result.